@ZH Rome
Gaius Marius
I like it, very synergistic with a nice emphasis on experience. That said it could potentially get a little OP - if you've got the 3 experience buildings in the capital, and say, 10 other cities also with 3 experience buildings, all of your units would start with 75 experience. Odlly, this'd make Marius most powerful in the lategame, which may or may not be a problem overall. That said, the legionary is great, simple and works with the UA and I love the arena, which, again, works great with the UA.
Caesar
This could be a little overpowered - starting a We Love the King Day in
every city whenever you take a city could put you in a perpetual WLTKD if you're strong enough. That gets a little OP when combined with the unique general building lots of free stuff for you. And on the reverse, if you're really strong and facing a weak foe, it'd force you to take a city every 5 turns or you'd miss out on the next WLTKD. That said its still an interesting UA/UU - though I don't see that the UB fits very well. It'll only really be useful when you're on the defensive, and more importantly, the secondary ability relies on you being in WLTKD a lot, which is weird because you get the WLTKD by playing offensively, whereas the bonus is defensive. I can see that secondary bonus being totally unused 99% of games.
Augustus
I like this one. Looks fun to play and fairly powerful and is also a cool way to split down vanilla Romes UA/UU.
Trajan
I think the second bit is a little confusing to be honest. Other than that, I like it! Though if the additional maintenance for the UB is too high, it could be really annoying, since monuments tend to be something you build in every city, and the rest of the civ has no gold generation bonuses.
Constantine
Depending on how much faith it costs, the Archdiocese thing could be really overpowered. I'd reccomend that the price of creating one increases for each already in the Empire. Other than that I've my usual quibbles about new mechanics rather than using the existing mechanics, but I know thats just a personal difference of opinion. Other than that, I like it!
Theodosius
As usual, I think the foederatii is cheating a bit since its a 'hidden' unique. I'd rather see it integrated into the whole design as a UI like JFDs Germans. I think the basilica seems overpowered. I build temples in pretty much every city - so that basically means a constant +50% culture modifier across the whole empire.
Nero
The flat gold and production cost decrease seems a little odd, it doesn't really fit the rest of the design and is rather boring if you ask me. The Horseman seems pretty overpowered because one could potentially just sell 5 shrines or monuments and rebuild them, meaning +5 happiness per city. Plus I'm not sure this civ is as synergistic as your others. There isn't much cyclicalism because theres no bonus for happiness and the theatre granting culture goes against the rest of the civs design a bit.
Marcus Aurelius
This one is a tad odd because my first instinct would be to carpet the world in scouts and rake in an absolute tonne of GW points. That said the milites and the UA synergise well together, and the UB works with the whole thing very well. Only quibble I have is that 'Roman circus' seems like an odd name for a UB replacing the circus.
):growth and
great people genration in conquered cities by 5%.
Congratulations.
Technology, and provide
Artifact in the nearest city, unlike the
Production for every enemy Spy ever captured, unlike the National Intelligence Agency it replaces.
Combat Strength for every
conquering a city provides points towards a 
religion do not reduce piety nor produce religious pressure.
ranaged units have a 33% bonus against naval units. after reaserching the required technologies, receive gunpowder for free.
culture and 
movement and sight. cost more to 
