If Blues succeed in the early game, they mass foxhole snipers and then green cant even try crossing the river without dieing. If green Rushes and the map ends up being relatively evenly distributed, green cant be stopped from tunneling.
If blues can take the mountains that are right beside the river, they can forever snipe and basically win due to ridiculous terrain OP.
If greens can take the mountains then they can tunnel from anywhere easily & they are on even terrain with the blues.

How to fix, make both team start on mountain, with a river/lowlands in between the two. More epic slow trench warfare with more tactics etc.

no.

the uneven-ness is part of the appeal of the game

Actually it really is quite easy if you don't stress over the intel, the river can be a problem and it can turn into a stalemate but with enough teamwork you can easily build a bridge and burrow into the mountain to avoid sniper fire. From there you just set up a more secure way across.

It adds to the appeal of the game and makes it alot more tense, if you always have the high ground it would be much too easy - the difficulty adds alot to the game.

I agree with Angus.

Green can destroy those crapholes quite easily. Just ran in them from side, take out your shovel and destroy all possible sniping positions. And blues can't snipe forever. I shown them many times they can't.

And tunnels work in both ways you know.