I have a soft spot for the Research Institute... yes, it comes horribly late but the bonus is glorious once you actually get it and it is reasonably beelinable. If having one of the most popular great wonders as a throw-in doesn't sound good, maybe the following does.
Free specialists don't push your growth caps, so ignoring effects from city size (maintenance and civic upkeep vs. trade routes and free unit upkeep; could go either way but bigger certainly isn't better if you have well-stocked corporations) the bonus from the Research Institute is worth:
2 scientist slots, +4

, +2

, +2

.
Now, getting that food surplus would normally require 2 grassland farms... and of course, the citizens working them would count against the growth caps too. So we end up with
2 scientist slots, magically grows your BFC by 2 flat grassland tiles, +4

, +4
This sounds like a rather desirable piece of infrastructure to me. Still not good enough to be worth the wait? Maybe not... but in my opinion the other lategame UBs don't offer nearly as much to compensate.
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The forum is easily identifiable as weak. 4 times the bonus of almost any other UB (if scalable) instead of a trait would be too powerful, 4 times the bonus of the forum is strictly worse than PHI.
Maybe not among the very weakest in the real world because it comes reasonably early, doesn't obsolete and it will reliably have a minor effect (things like the Obelisk and most military UBs can be completely irrelevant)... but certainly among the least powerful.
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The Assembly Plant is an interesting one. In my opinion it's weak overall and downright terrible for Frederick.
In addition to the engineer slots, the discount for coal puts you 86 final hammers ahead... unless you get the ORG discount in which case you're only 30 final hammers ahead.
You can spend these hammers however you wish (good since you probably have specific and urgent needs at this time, like coal plants, health buildings or hordes of infantry) but unlike other UBs the benefit doesn't increase the potential of a fully developed city (disappointing if you've been waiting for that long).
In a competition of the late bloomers, I think I prefer the Shale Plant and Mall, and I definitely rate the Research Institute above it.