Final Frontier and GalCiv aren't at all similar. I think the turn limit in the demo did a good job hiding how well.. limited the mod really is. Five types of ships, a couple more if you count fighters and bombers. Each ship has about four or five upgrades. I like that they made new perks for ships that gained XP, although they pretty much amounted to the same thing the regular game's perks did.
I didn't feel the mod had any longevity is my problem with it. I didn't even bother finishing it the one time I tried it out since buying the expansion. It seems to me that it will basically play out the same every time I try it out because it doesn't have different eras like Civ IV. There's no great transition in how to use units, a Delta Destroyer is better than an Alpha Destroyer (early unit), but they operate the same.
Each city being a solar system and each planet acquiring a different assortment of buildings is sorta neat, but it felt loose to me and hard to manage. It is just complex enough that it may allow for some deep strategy if I take the time to really learn the ins and outs, and I do like that.
I did like how fighters and bombers are made useful, but vulnerable. There's some cheap units (destroyers) that are pretty decent at intercepting them, but fighters and bombers can actually destroy ships in Final Frontier which makes it worthwhile to use them.
I really wish they'd done the same in the Pacific War mod that comes with BTS.. yeah, aircraft can't sink ships in the Pacific war, the CARRIER war, the AIR war.
Of course, I do have to compliment them on using easily accessable scripting for all unit stats and capabilities and whatnot. After some trial and error I was able to make a mod that upped all units' air-interception ability, and allowed air units to either heavily deplete or kill all sorts of units. So it's not so bad.
I also have to give kudos for the AI.. I'm not totally sure if it was Warlords or BTS, but even just on a low level of difficulty it's ruthless. Doesn't matter how much they like you anymore, if they believe they can get away with invading and taking a city or two on the first or second turn of declaring war.. they will. Sure it can be irritating not knowing who'll stab you in the back, but it's EXACTLY how I play. If I think I can get a leg up by attacking my best AI friend then I will, so how can I fault them?