It's not a matter of reliability but emphasis. @Noname Given is explaining it better than I, but Kirk in this alternate take is not in the special class of Starship captains, and lacks the experience that would understandably go along with it. To me, different aspects of the personality get emphasis or priority depending on the job. When I worked retail I was a go to it type of person who was expected to get projects done a certain way. At my current job, as a supervisor, it looks way different and those aspects of my personality do not get emphasized as much.He might arguably adapt to the realities of his crew, but that would not explain why he was so different with Pike. If he would be so impressionable with each crew that his personality would change from posting to posting, he would be unreliable.
I'm not explaining this as well as I would like but I don't think it is personality but approach in interacting with people, which is shaped by the demands of a job and people we interact with. At least in my experience.
Russ did a lot more with a Vulcan than Spock did, and it was really neat to see that. Russ provided this idea of extreme privacy in his Vulcan portrayal, yet underneath that all was a lot of emotion. Him and Nimoy are my go to Vulcans, with Lenard, and Quinto right behind them.Frankly, Tuvok was as good a "Spock" as any Spock since Nimoy. I felt Russ's Tuvok, captured more of Nimoy's original Spock than any other actual "Spock" actor. Of course, the argument follows that perhaps that's not what Russ should've done, but..., just sayin.