A point of contention in this one is why Spock would struggle with his human emotions, when Vulcan emotions are supposed to be "stronger." I don't think there's been any suggestion in the past that emotional control is a Vulcan genetic trait.
This is given some cover within the episode, when Spock vaguely says that the human emotions are "different," but he doesn't elaborate.
I wonder if, in keeping with Spock's character in the franchise as a whole and in this series in particular, Spock's predicament allowed him to subconsciously give himself "permission" to embrace and explore his emotional side, with the genetic alteration as an excuse.
I kind of like the idea that Vulcans' control of their emotions is a genetic trait. It doesn't seem too far-fetched, since Vulcans already have mental abilities humans don't, and it gives a bit of an in-universe reason for why Star Trek writers seem really inconsistent on whether Vulcans have emotions or not.
I've always thought the Romulans weren't just the Vulcans who rejected Surak's teachings, but also any who didn't have the physical ability to follow them.
Originally it was just based on Romulans not expressing any psychic abilities, but Picard also established that sharing personal details publicly as a major taboo. That would track with them being a mix of former enemies who are concerned with suppressing the rivalries that lead to nuclear war.