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.
Unique building for the Russians; Replaces Laboratory
+50% spaceship production
+2 free scientists
Can turn 1 citizen into scientist
This is a laboratory that provides you with 2 free specialists in the form of scientists. This is huge. By itself - imagine chosing whatever unique building for your first city in an ancient start - this is the strongest building in the game. It still sucks and is basically unplayable with maybe an exception in certain OneCityChallenge (OCC) setups. Why is that ?
Two free scientists provide you with additional 3 base technology (6 if you are in Representation) and 3 GreatScientist base GP-points each. The first problem is that laboratories come very late in the game, even if you rush on them - and rushing them delays other stuff. This results in a comparatively short time during which you are actually going to use them, no matter whether the game ends on time, gets called on points or finished with a space ship.
The second and main problem though is not the relatively short time you are going to use it, but that all strategies which would profit from a Research Institute - mainly Space Race - profit from other buildings (much) more. For example simply having +2 from a Hammam in the late game just enables you to work those 2 specialists anyway, but also over the course of most of the game is very useful.
Summary: The Research Institute´s effect is very strong in comparison with other buildings, but it comes too late in the game to be useful in any scenario (maybe Industrial start OCC) - or in other words, the effect is too weak compared to its timing both in Ironman or Industrial start team games.
Unique building for the Incans; Replaces Granary
Stores 50% of after growth
+1 from Corn, Rice, Wheat
Available for free on Medieval and later starts
This is one of the best buildings for any game that starts in ancient or classical and still decent in medieval. The main use of Incans and their UB+UU is of course an ancient start teamer, CTON or Ironman. Granaries are by themselves one of the most important buildings in any settings. The Terrace provides half a Creative trait, making building monuments unnecessary. This are early 20 you save for each monument. Additionaly the Terrace enables a player to plant his first cities more flexible, not having to make sure the city´s food or strategical ressource is in the inner fatcross to be able to work it right away since usually building a monument and waiting 5 turns for borders to expand would delay that.
Summary: The terrace is a granary, which you most of the time want to build as the first building in a city anyway, now you always can (never monument first) - and it´s better then a monument, too with its +2 instead of +1 . City planting is more flexible and most important of all: It saves early hammers.
Edit: Just realized how good that +2 instead of +1 actually is. It gets you a second border expansion (40% defense!) just in time against a first big stack instead of just against a theoretical second one unlike a monument which needs 50 instead of 25 turns to achieve that!