So are Rome really that good civ to be considered Uppet Tier. In the same tier as Austria and Ethiopia? Yes, their UA is flexible and stuff. But their UUs are not very good. I loved playing Rome in Vanilla with the march style where you just build billions ballistas and legions, walk in a straight line and smash everything under the armored foots of the legions sieging the AIs caps, taking them in 2 turns in the early game (building roads for your ballistas at same time). They are suited best for playing wide and then switch to playing tall using your UA and by doing this getting not only 1 or 2 great cities but getting 4+ great cities. I cannot think of any other way playing them at their best.
I didn't yet played them in BNW but i don't think there is a big change at their playstyle since vanilla and that is 2 expansions.
Play a game and see. Don't attack anyone. Don't build more than a couple of legion (like, literally 2, and don't upgrade them until post-railroads). Ignore the ballista. Go liberty and found 6-8 cities by mid-game (they don't have to all be great cities, just make sure your happiness is keeping up). Use food/hammer routes liberally in mid-game. Gold-buy every science building in the capital (and anything else you want in all your cities fast). Or, even if you go tradition, Rome's UA will be valuable enough to justify it's spot. Just go by how smooth things feel at the end. Unless you are very meticulous about the timing of each tech/building, you won't notice anything. You'll just think things are going well.... always. But the math shows Rome's UA's power. See below:
The problem with BNW is that hammers have become scarce in relation to what must be built. In Vanilla, you could build everything in a tall empire which pursuing the most science-route using hammers in a Tradition 4-city build. In G&K, you start to run into some issues where your science is outpacing your production. But, that's fine, there're some buildings you don't need anyway. In BNW, there are a ton of buildings, almost all of which are useful and have compounded values (so, you want to get them asap). For most BNW civs, this means you'll be using gold to buy some buildings. But, gold is SUPER inefficient. To give you an idea of how awful gold-buying is.... Universities cost 160 hammers, or 660 gold. That's more than 4x the amount of gold!!!!! Keep in mind that to generate gold from tiles, it's usually a 1:1 tradeoff. This is why the AI in "default" city management settings always prioritizes gold dead last. Gold is both the hardest to get (especially in the early game), and the most worthless thing to use. Only at the very end-game, is gold so plentiful that you'll consider a 4:1 tradeoff to be fair value (mostly because you have no idea where else to spend it). Once you commit to "I don't run out of useful things to build until the very end of the game", Rome's UA becomes a direct 4:1 savings versus a gold bonus. Even better, Rome can just generate more value by plopping down another city, whereas most other UAs are locked to a set value (Portugal to # of trade routes; Greece to # of city states).
Anyway, when you follow the math, ANYTHING in BNW that saves hammers early/mid-game, is ridiculously important, and even more important on deity because you're teching at a faster rate. Rome's UA (4:1 important) is less impressive if you have 400 turns to hit the end of the tech tree, versus 250/300 turns.
Rome doesn't just save hammers, it saves an insane amount of hammers. Consider a comparison with Greece. Greece saves 50% of CS-directed gold. And, even in non-Greece games, you DO run out of CSes that need gold. It's not really a never-ending pit. Greece is a good comparison for value because it is also a "savings" type UA (whereas, with a pure gold bonus like Portugal, the flexibility has a certain value). In this case, Rome's 25% savings is actually
twice as good as Greece's 50% savings, due to the natural 4:1 conversion rate... for all cities you run out of stuff to build (in a 4-city tradition, this means it's something like 150% the value of Greece's UA, not actually twice). In reality, even with Rome's hammer discount, you won't be able to build everything. And the discount gets really ridiculous when you have 6-8 cities, especially when obtained through peaceful spread. (Of course, conquest will ultimately generate more bonuses from the sheer number of cities you'll take and use, but it's less good on a per-city calculation).
Side note:
This is also why everything that saves or gives hammers is extra extra valuable (along with food, but that's more obvious), and everything that saves gold is not worth as much as it seems. This is not a "Civilization V" thing, this only happened in BNW. Before, in G&K and especially Vanilla, hammers were only extra valuable if you need to build units, because otherwise, you could more or less build everything you wanted. In BNW, your 4th city will be lucky to have generated half the number of hammers it needs to build everything you want. So, everything hammer-related got much better in BNW (to the degree of hammers saved/granted), including anything that allows you to trade off food on tiles. Direct Hammers: Poland, Egypt, Rome, Persia, Russia, Germany, Huns, Carthage, Iroquois. Food-tradeoff hammers: Inca, Siam, Netherlands, Aztec.
The low-tiered civs on this list have difficult to utilize hammers (for example, Iroquois may actually lose hammers in certain cities and during certain eras; Huns/Carthage gain/save very few hammers; Germany's hammer bonus doesn't really kick in until mid-late game, and it has a significant gold-opportunity cost). Being forced to create units also essentially raises the total hammer cost for your strategy, so aggressive civs need more hammers to begin with, and some of these UAs merely offset the need, by however much.