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

Forum
Unique building for Rome; Replaces MarketThis is a market that provides a 25% bonus for the birth of Great People. While this sounds like a significant bonus at first, it really is not in any strategy that circles around or involves a focus on getting Great People. That is because any such strategy takes advantage of all or a combination of the Philosphical Trait (+100% on GPs), of the Pacifism Civic (+100%), of the National Epic (+100%) and of the Golden Age (+100%). An additional 25% are insignifanct - they do not result in any additional Great People, even over the course of a long game like an Ironman.

Also take into account that the market is a very low priority, but expensive building (100 on quick game speed) that only finds some use (mostly for its additional happiness) in Ironman games. Whenever Great People play a role, much more significant bonuses are available without the additional big investment of 100.

Good

Baray
Unique building for Khmer; Replaces AqueductThe baray is an aqueduct that provides an additional food to the city that builds it and especially that gets it for free in Renaissance and later then that era starts. The key aspect is the "free on Renaissance and later starts". Mainly in Renaissance and Industrial games the baray is equivalent to a bonus on buildup, quite similar to an Expansive or an Imperialistic trait. Initial cities grow (and regrow after slaving [or drafting]) faster, new cities get going better.

A simple practical example - imagine you planted your first city in a Renaissance start game, it is size two. You work two 3-food tiles (for example a flood plain and an unimproved corn), that is a surplus of four food. The baray does 25% of that just like that. You probably slave down those cities at first to size 1 for workers for example. Even if you improve a tile (which you probably don´t do till a couple of turns into the game since you are probably chopping forests at first) the baray still provides a significant increase of the food your city accumulates. Overall Khmer´s cities grow sligthly, but significantly enough to matter faster.

Also, the baray makes Khmer the number one pick in OneCityChallenge (OCC) Renaissance (and probably Industrial) games. You grow faster initially (faster initial research allowing to gain an advantage from the start) and in the end can most often grow your city one size bigger then non-Khmer opponents.

The baray is one of the few unique buildings - next to the Dike (hammers), the Feitoria (commerce) and the Madrasa (culture) - that provides an absolute, not percentual bonus on economical aspects where usually none exists.

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