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.
I think Vulcan brains are fundamentally different in their ability to (almost) completely block out emotional reactions, but with those barriers lowered they have significantly more difficulty keeping themselves together than humans do. I am told that a human who tried to go "full Vulcan" with their emotions would mess themselves up pretty badly, but Vulcans manage a functional society of mostly decent people doing that.
Spock, therefore, would have found himself experiencing less overwhelming emotional urges than he experienced in his occasional moments of lost control, but with none of the vulcan-specific mental barriers he was accustomed to leaning on he had little choice but to roll with them anyway.
If those Vulcan mental barriers are variable in effectiveness between different individuals and different emotions, that would also present a needlessly technical explanation for why various Vulcan characters fall into obviously emotional patterns while maintaining very Vulcan outward behavior in other facets. Captain Solok and his racist vendeta as an obvious example.