In my first game of CPP (so take this with a grain of salt), I think I have to agree the UA feels a bit too strong.
With a standard Pangea map, I started near an enormous clump of mountains (enough for 6 cities settled on mountains without many overlapping tiles). The UA allowed founding cities in somewhat ridiculous locations (like on a mountain surrounded by two rings of mountains). AIs can't found cities in such mountainous places, and that is a huge advantage in itself.
The other advantage is of course that massive 'for every 2 mountains' yields from the Pantheon belief (and lesser extent observatory).
The sea of +3G/+3F tiles in the mountain clump seemed odd, as uniformity like this is rare for other terrains (due to forests, bonus yields etc.). Also why are mountains such food powerhouses, shouldn't they be more hill-like and grant production?
Regardless of the above, the UA mountain-walking and settling was extremely fun. I would suggest only minor tweaks to yields and perhaps the Pantheon belief.
For sure.
What if lone mountains were the stronger tiles and mountain ranges diluted the yields, since that's where the super farms are going to inevitably be? It also makes sure that cities that can go nuts from the mountain pantheon also don't have a bunch of the best tiles in early game to match. Just a thought.
I like the idea of making mountains better where terrace farms are worse.
My idea would be:
"Mountain tiles produce +2P, +1F/+1G for every two adjacent non-mountain tiles and +1P for every two adjacent mountain tiles".
e.g. tile next to:
0 mountains: mountain 3F/3G/2P (amazing), terrace farm bonus n/a
1 mountain: mountain 2F/2G/2P (bad), terrace farm bonus +1F (bad)
2 mountains: mountain 2F/2G/3P (good), terrace farm bonus +2F (ok)
3 mountains: mountain 1F/1G/3P (bad), terrace farm bonus +3F (good)
4 mountains: mountain 1F/1G/4P (good), terrace farm bonus +4F (amazing)
5 mountains: mountain 4P (bad), terrace farm bonus +5F (amazing)
6 mountains: mountain 5P (ok), terrace farm bonus +6F (amazing)
Other miscellaneous comments:
1. I was surprised my enemy could use my mountain roads to invade me (I deliberately routed my roads on mountains tiles where I could). Lesson learned for next time, as I guess it makes some sense.
2. It felt weird to have double movement on mountains - it would make more sense to have double movement on hills and single movement on mountains (i.e. mountains are the new hills).