Game is pretty buggy with LOS. Quite a few times I'm half way around a pillar and get hit by a cast that's ~75% done by the time I'm out of LOS. Happens on other players' screens so it's not just a disconnect from server and client side.
Unless talented, Vanguard interrupt(just for example) is 12 seconds, providing the Merc/Commando with 8 seconds of free casting.
It's not that they're really that OP, it's just it's such a low skill cap for high returns.
Just buff interrupts(would promote players to play better), and increase the skill required to pump out good DPS as a merc/commando.