And the only TOS cast members who gave good performances in TAS were those with prior voice acting experience, like Doohan and Takei (and Nichols through her singing experience). The production and the recording process were so rushed that the actors didn't really get a lot of direction, so those without voice experience didn't have any guidance in how to do it effectively.
It's pretty noticeable if we compare the original TAS version and the french(-canadian) one. For example, the guy who did Kirk's voice is one of the greatest Quebec's actors and he did a lot of dubbing. So, he made a better job than Shatner and the recording is less variable.