The best way to find out about the weaknesses of a particular class is to play said class yourself.
I thought transmuters were OP until I played one, etc.
I haven't played a monk before (or any paladin to be honest) so I don't really know how OP they are or not but...
- you're spot on with the kill sealing ability which they lack,
- keeping all those nice sups up all the time also probably requires a lot of mana, if you can do anything about it,
- maybe a hit and run tactic with heals in between until they run out of mana could be an interesting strategy? (for a melee class),
- with an assassin and transmuter you have one of the most powerful weapon in the game which is the element of surprise.
Just a few thoughts.