It depends on how it works. pokoko makes a good point that if its implemented poorly, ability to max everything out could make you overpowered.
The way Monster Hunter does it, you can specialize in *everything*, but it would take an insane amount of work, and you can only have one set of armor and weapons active at a time. If you had to "switch" between active skill trees, and could only do so outside battle, it's less OP than just becoming a functional god.
Monster Hunter: pissing me off since 2010.