He's being too hard on himself. If you compare the completely wooden acting styles of Keanu Reeves, Kevin Costner and a ton more - William Shatner absolutely shines. I quite like what he brought to the series as much as I like the series itself. Star Trek:TOS wouldn't have been the classic it IS without him.

Anyway, there are many, many actors who don't view their own work and are highly critical of themselves for whatever reasons. His career hasn't suffered because of it.