There's nothing that makes classless inherently more complicated or unbalanced than classes-- something like Fate is simple, classless and balanced, while 3.5 D&D is classed and...well, we all know how that one goes. The core distinction is that a class-based system pushes characters into filling certain archetypes (Powered by the Apocalypse systems are a really pure example of this). That's not inherently a bad thing, but it is a distinctive thing-- is that something you want in your game?