I thought he was godawful in UNIFICATION, but part of that is having two recessive performers on screen, and Stewart/Nimoy makes me think Hunter/Nimoy would never have worked in the dynamic compelling way Shatner/Nimoy does
Leonard actually talks about this a bit in Mind Meld
. He says because Jeff Hunter gave a much more 'internalized' performance, Leonard was being more extroverted to create a contrast. He thought what Shatner brought to the table, with a more extrovert Captain, allowed him to internalize Spock and add that Vulcan mysticism.
I also agree that he seems straight-jacketed in his performance in UNIFICATION. He pretty much only agreed to do it as a cross-promotion to STVI, but it's obvious that he can't quite figure out how to find a chemistry between his character and Picard/Data. It kind of works for the story being told, Spock is a little aloof and distant anyway. But I've always thought Nimoy looks pretty uncomfortable at times throughout.