you raise a good point about planeswalkers. i'm still forget often that anything that hits a player can hit a planeswalker atleast as far as lighting bolts and other player target abilitys are concerned.
only one i can think of shooting in tezzeret but hes never out when i have trisky out (as of yet hasn't happened.)
Moreover, that it's not a beater unless they have critters to kill is a major minus. Having played quite a bit of MonoBrown, you'll often rely on a single beater to go all the way, and losing out on 2-3 of those in your deck is simply too much to lose.