One of the criticisms that I read of Shatner was that at the end of the day, instead of hanging out with his fellow actors, he would leave the studio and spend time with his established friends and his family. He treated his fellow actors as co-workers and not his new best friends.
Star Trek wasn't a modern ensemble cast show. It had three primary stars, Doohan was medium sized and the rest were in supporting roles with minimal scripted lines. Nickels wasn't even a contract player, she was a day player, basically waited to see if she was on the daily call sheet.
Shatner isn't the best actor I've ever seen. But he is considerably above average in terms of his acting ability, his career shows this. With the exception of Nimoy, the majority of the Star Trek TOS cast has worked infrequently.