Yeah, you might find having mods installed in one location but not the other or having outdated mods totally stuffs up your game. That's why you get errors.
I have a few mods installed for minecraft 1.7.10, they have all been updated and I can't find anything like the legacy java fixer to solve this. When I click play it just closes the launcher and (this is just my settings that do this) immediately re-opens it :'(. The log it gives me is:
Somebody Please Help
-Pixstatic