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Even though the title suggests different, there are basically no just good or just bad unique buildings (and units). Every UB and UU has to be evaluated in the context of the game settings and overall strategy it´s supposed to be used in. A (fictional) unique unit archer that gets +100% against melee units is useless in an game played on Islands, where you have no (military) contact with your opponents until Astronomy is researched and archers are long obsolete. A unique building is good if it significantly supports a (playable) strategy.
Bad
Assembly Plant
Unique building for Germany; Replaces Factory
+1
; +2 from Oil and Coal
Can turn 4 citizens into Engineers
+25%
(+50% with Power)
+50% with Coal
The Assembly Plant is a factory that receives a 50% production bonus from coal and that lets a player work four Engineer specialists instead of two. The main reason to want to be able to work Engineers is the wish of getting a Great Engineer which can be used in order to build one of the corporations Mining Inc. / Creative Constructions. Those are gone in any slightly competetive game once Assembly Line, the prerequisite for Factories, is researched. In any case you cannot research Assembly Line and only then go for the corporations if you want to have a chance of getting them.
It is very rarely a good idea (or sign of a not optimal buildup) to work additional engineer specialists instead of tiles if the goal is increasing production of a city. Those very few instances don´t justify picking for the Assembly Plant.
Finally there is the additional +50% with Coal. At the point in a game where you build factories, city production is already high and numerous other bonuses are available. In an Ironman you have the forge, you have an additional hammer from Universal Suffrage on your towns, potential levees, maybe a Golden Age increasing all those hammers from towns by 100% from 1 to 2 (a Golden Age at this point is very common in an Ironman), railrods on mines, corporations, you might even be Organized (double production for factories) etc. In a modern or future start game, you run a workshop economy anyway, having vast production with Caste System and State Property - so a factory is done in a turn or two anyway, not mentioning that in such a scenario you even have the option of putting a chop into it as well.
In any case, the additional 50% with coal doesn´t do much, at best safe you a few turns in an Ironman like game (2-3 might already be optimistic). Such a gain is meaningless seeing how late this building occurs in the game.
Good
Mausoleum
Unique building for the Indians; Replaces Jail
+4
; +50%
+2
-25% War
Can turn 2 citizens into spy
Hammam
Unique building for Ottomans; Replaces Aqueduct
+2
+2
Ball Court
Unique building for the Maya; Replaces Colosseum
+3
+1 per 20% 
rate
These are a colloseum, an aqueduct and a jail that additionaly to what they usually do provide an extra +2 . This makes their three civs belong to the absolute top of picks in an Ironman / ffa game (click here for an article on game types in civ multiplayer). The reason being that while running a Cottage Economy (which is basically always, though not necessary solely) and in later game changing to civics "Universal Suffrage" + "Free Speech" + "Emancipation" etc. confronts you with a happines cap (max amount of population you can have till you get unhappy citizens) that is the main limiting factor of how big your cities can become. Those three buildings increase that cap by two, resulting in 20 or so cities being able to grow two population further each - that´s a lot of additional production, commerce and points.
The main differences between the three lie in the nature of the buildings they are replacing - though the main argument for which you pick in an Ironman rather being the starting techs of the civ and additionaly which Unique Unit they provide.
The Mausoleum (jail) comes rather late and is not a must have in every city, but in another way of argumenting you get +2 and potentially some advantage on spy points you´d otherwise not have seem worth getting for the needed investment.
The Ball Court requires Construction, which in many (most...) Ironman games is a technology with low priority you avoid for a very long time and start to want only if you have to defend against an attack and require catapults or finally want to attack your opponent, sometimes never. Since Ironman games though are in their nature games where you most of the time have to build up your empire first (till around guilds the first big attack waves come), you don´t want to tech Construction too soon. Not a big argument against the Ball Court, but plays a role more often then you´d think.
The Hammam is an aqueduct which you get with Mathematics, which is very high priority. Aqueducts you build anyway and together with their usual +2 make the Hammam crawl up in the build order even a little bit more. They come early (though aren´t required necessarily too early), come with a must have technology and the building they replace is a must build in Ironman anyway.
Overall Ottomans have great starting techs (and Janissary are nice, too in Ironman), India has the Fast Worker and Maya the Holkan for being safe early against an opponent sending a couple of chariots. For a long game of civ without a city elimination with lots of buildup it probably goes 1. India 2. Ottomans 3. Maya. Besides that there aren´t many other civilizations that can compete with those three as picks in such a scenario.
For Good UB - Bad UB part #1 click here
For Good UB - Bad UB part #2 click here
For Good UB - Bad UB part #3 click here
For Good UB - Bad UB part #4 click here
For Good UB - Bad UB part #5 click here
Bad
Assembly Plant
Unique building for Germany; Replaces Factory
+1
; +2 from Oil and CoalCan turn 4 citizens into Engineers
+25%
(+50% with Power)+50%
with CoalIt is very rarely a good idea (or sign of a not optimal buildup) to work additional engineer specialists instead of tiles if the goal is increasing production of a city. Those very few instances don´t justify picking for the Assembly Plant.
Finally there is the additional +50%
with Coal. At the point in a game where you build factories, city production is already high and numerous other bonuses are available. In an Ironman you have the forge, you have an additional hammer from Universal Suffrage on your towns, potential levees, maybe a Golden Age increasing all those hammers from towns by 100% from 1 to 2 (a Golden Age at this point is very common in an Ironman), railrods on mines, corporations, you might even be Organized (double production for factories) etc. In a modern or future start game, you run a workshop economy anyway, having vast production with Caste System and State Property - so a factory is done in a turn or two anyway, not mentioning that in such a scenario you even have the option of putting a chop into it as well.In any case, the additional 50%
with coal doesn´t do much, at best safe you a few turns in an Ironman like game (2-3 might already be optimistic). Such a gain is meaningless seeing how late this building occurs in the game.Good
Mausoleum
Unique building for the Indians; Replaces Jail
+4
; +50% +2
-25% War
Can turn 2 citizens into spy
Unique building for Ottomans; Replaces Aqueduct
+2
+2
Unique building for the Maya; Replaces Colosseum
+3
+1
per 20% rate. This makes their three civs belong to the absolute top of picks in an Ironman / ffa game (click here for an article on game types in civ multiplayer). The reason being that while running a Cottage Economy (which is basically always, though not necessary solely) and in later game changing to civics "Universal Suffrage" + "Free Speech" + "Emancipation" etc. confronts you with a happines cap (max amount of population you can have till you get unhappy citizens) that is the main limiting factor of how big your cities can become. Those three buildings increase that cap by two, resulting in 20 or so cities being able to grow two population further each - that´s a lot of additional production, commerce and points.The main differences between the three lie in the nature of the buildings they are replacing - though the main argument for which you pick in an Ironman rather being the starting techs of the civ and additionaly which Unique Unit they provide.
The Mausoleum (jail) comes rather late and is not a must have in every city, but in another way of argumenting you get +2
and potentially some advantage on spy points you´d otherwise not have seem worth getting for the needed investment.The Ball Court requires Construction, which in many (most...) Ironman games is a technology with low priority you avoid for a very long time and start to want only if you have to defend against an attack and require catapults or finally want to attack your opponent, sometimes never. Since Ironman games though are in their nature games where you most of the time have to build up your empire first (till around guilds the first big attack waves come), you don´t want to tech Construction too soon. Not a big argument against the Ball Court, but plays a role more often then you´d think.
The Hammam is an aqueduct which you get with Mathematics, which is very high priority. Aqueducts you build anyway and together with their usual +2
make the Hammam crawl up in the build order even a little bit more. They come early (though aren´t required necessarily too early), come with a must have technology and the building they replace is a must build in Ironman anyway.Overall Ottomans have great starting techs (and Janissary are nice, too in Ironman), India has the Fast Worker and Maya the Holkan for being safe early against an opponent sending a couple of chariots. For a long game of civ without a city elimination with lots of buildup it probably goes 1. India 2. Ottomans 3. Maya. Besides that there aren´t many other civilizations that can compete with those three as picks in such a scenario.
For Good UB - Bad UB part #1 click here
For Good UB - Bad UB part #2 click here
For Good UB - Bad UB part #3 click here
For Good UB - Bad UB part #4 click here
For Good UB - Bad UB part #5 click here