While I think many UAs could stand with some minor tweaks (I've covered some of them in other posts), either because they need to be improved a bit or because they merely don't sync with my play style as much as I'd want them to, the most
egregious UAs (i.e. the ones that really just suck) are:
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India
Why it needs to change: This one takes the cake. It makes India suited to only one play style (small and tall) and just seems very generic. I say generic because it never really involves the player in any way. It's just there. And it's one of the few (the only one?) that has a negative modifier attached. Furthermore, it seems slightly offensive to characterize India by population growth. Thousands upon thousands of years of history and...
population growth is what we get.
How I'd change it: sure, okay, India has a big population. They're populace. Maybe fold that in more discretely if you're set on having that element. Here's my alternate UA:
Sacred Rivers: Cities adjacent to a river receive +1

faith and +10%

growth.
There. Built in growth mechanic to actually help get that big population and a faith element (because how many religions came out of India?). Combine that faith with an early "Sacred Waters" pantheon belief, and you've got a growing, happy river populace. Much more in line with India historically and a little more engaging.
I justify it because India has many sacred rivers flowing through it: the Ganges, Yamuna, Sarasvati, Narmada, etc. Heck, Gandhi is standing on the banks of the Ganges (at least I think that's what it's supposed to be. Might be a beach in Brazil).
The Celts
Why it needs to change: I think this UA (and the one following it) were both conceived before Firaxis had a good handle on how powerful the religions in the game were. They were still being cautious with its application and of giving too much to the civilizations who had faith based UAs (further testament is the fact that the Pictish Warrior loses his ability to gain faith from kills). Honestly, the Celts and Byzantine UAs feel like two parts of what should be
one UA, one that both guarantees you a religion and gives you an extra belief.
The problem with the Celt UA is just how
weak it is. Very conditional. First, you must have a city directly next to a forest to get any bonus. Second, in order to get the bonus, you cannot improve the forest tile
in any way. Third, you only get the extra faith if you have
three adjacent forest tiles. For three, you only get +1

faith.
"But the Celts are guaranteed to found a pantheon first!" Okay, yeah, 99 times out of 100, they'll pick the first pantheon belief.
So what? They definitely aren't guaranteed to found the first religion, which is much more important, in my opinion.
How I'd change it: I don't have a specific change for this one. It could be improved numerous ways. They could change it to where you can improve the forest tiles for one, because I'm sorry, but it's just
slightly ironic to lose faith in a tile I build a camp to hunt deer in if my pantheon is Goddess of the Hunt. They could also change the forest tiles to produce culture instead of faith after you found a pantheon or religion. I don't know. Just something.
The Byzantines
Why it needs to change: Lastly, Theodora. Great UA really. Makes your religion very versatile. But what if, just what
if, despite your best efforts, you fail to found a religion? Uh... well, there goes that UA.
How I'd change it: Give them some sort of backup effect. Maybe they get the full effect of whatever religion comes to dominate their capital if they fail to found their own. Or perhaps add in some sort of faith bonus (like the Celts) that virtually guarantees they'll be able to exercise their UA. Maybe they get to keep the effect of their pantheon no matter what (even if another religion dominates their cities).
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(Dis)honorable mentions: Germany. To a lesser degree, America and the Netherlands. I think the Netherlands is probably getting a buff this coming expansion anyway, sort of how England got one in G&Ks.
Fair enough explanations? Let me know what you think.