In general there are games that are easy to learn and hard to master as mastering them requires increase of skill; and games that are difficult to learn but mastery with them ends with learning your way around them.

With the first type is more diffucult to attract the wider gaming audience because an increase of skill requires perseverance at th face of own noobinesh and dedication. The second type of game can work as a world that sucks you in, and be more addictive and that's all the more easy to do on the pc.

Skill trees aren't bad in their own right, but they make more difficult to balance the game, as you need to do so for every possible combo of skills in one army against another.