It's ongoing and always evolving. I've started with humans and I do ok with some weapons more than others: rifle, shotgun, chaingun, pulse rifle, lasgun. The mass-driver, psaw and luci are probably the most difficult to master. Interesting is the fact that at first I couldn't see the deal with the shotgun and why it's so popular, as evidenced in my own earlier posts around this forum. That has changed.
I've only recently started playing alien more. At the moment I'm trying to improve my dretch and I'm finding unexpected joy in playing lisk. I have absolutely no skill worthy of mentioning at the others. The dretch is fast and has low HP. I have to practice hard in two areas in particular: wallwalk and walljumping, because I get severely disoriented sometimes and end up going the wrong way down a corridor or just madly wallwalking around as the world is spinning; and the final dance when approaching a human, that fine weaving at close distance to him. As for the lisk... it's the king of the lesser aliens. It can wallwalk, it can jump, it's slower than the dretch but has higher HP, and the special grab attack opens entire chapters on proper use and strategy. In fact, there is a
dedicated topic.
I'd venture to say that
all alien classes are hard to master. Which probably applies equally to (all) human weapons, but at least they're similar to other FPS games so the learning curve is smoother.
Builders and grangers have less to do with combat skills and more with strategy and making use of resources well. I'd say it's hard to choose between building for humans or for aliens, they're equally challenging.
One thing is for sure: as long as I find people to play with, I don't see myself losing interest in Trem anytime soon. It's simply too complex a game (which is something I never thought I'd say about a FPS).