I play casters a lot, and I came up with three ways to abuse the class as written within a minute of having read the full description. Arcanist might work in certain settings, but adjustments need to be made to things to accommodate them that in my opinion decrease the fun.
As for brawler replacing monk, that depends on the style of monk you're playing. It does mean you don't have to play a monk to be an effective pugilist which is good in my opinion. It's frustrating when you want to play a boxer and you end up having to choose to either be a really bad boxer or basically end up playing Jackie Chan. ;P
Slayer I think reads powerful, but would play weak. Shaman...yeah, I'll give you that one. It's on the upper end of the power scale for sure. I wouldn't necessarily want it in a RL game, but I think Avalon can handle it.