Nimoy's double stands out a bit in Mirror Mirror, in the Sickbay fight.
Mostly because his stunt double had wavy hair. WTF was that about?
The worst example of stunt doubling for Shatner was in Operation Annihilate when everyone was subduing Spock on the bridge. In the tight shots, it's Shatner, but in the long shots, Shatner is the only guy doubled and none of these moves are difficult. This leads me to believe Shatner might have been busy in another scene while this was being shot. Or maybe he was off.
I disagree that it was particularly normal for the time. I watch a lot of 60s television, mainly action shows in fact such as The Avengers, The Saint etc. I can think of no examples of a more obvious stunt double than in Space Seed.
You never saw The Fugitive, then. David Janssen's stunt doubles were always painfully obvious. Different hair, bone structure and Janssen couldn't run well (bad knee and drunk a lot), but his stunt guys ran like marathon runners. Voyage to the Bottom of the Sea had painfully
obvious stunt guys. What made it worse was that most of the stunt double were used as background crew, some with lines! So all of a sudden, David Hedison became some crewman named "Scott" (Scott McFadden) just in time to have a brawl!