He accomplished a QF once in his career. He made the 2nd week of a major 3 times in his career.
I'm downplaying nothing, just stating facts.
And I said nothing about Isner underachieving, so don't tell lies. If anything, he's overachieved.
My point is that you shouldn't need to add that qualifier. He is by far the single biggest overachiever on tour today. He may actually be the biggest overachiever in tennis history, at least if you compare their top rank to where they were at 16/17 years old.