Given that Scott just barely unseated Nelson, with the right Democrat it could flip. Nelson would have won if he'd ran a better campaign or retired.
Yeah Scott has had close elections all three times, and his margin of victory has declined in each one, what this indicates is he is a deeply polarizing figure, meaning if Republicans win FL by more than 2 points or so, he'll probably be fine, if it is within 2 points either way, it's probably a tossup, and if Democrats win it by 2 points or more, he has probably lost. Basically, I'd expect this to track very closely to the presidential and it is very possible that he underperforms the Republican presidential nominee in Florida.
Major difference, though: his gubernatorial elections were in GOP wave years. His election to the senate, however, was in a Democratic wave year against a three-term incumbent Senator.
Except it wasn't a wave year in the senate, and Democrats knew that going into the midterm.