She-Hulk VS Ant-Man
The other issue fans had was that She-Hulk was a complete outsider. The Fantastic Four were largely about family (an extended family, but still a family) and She-Hulk had almost zero history with any of the team members. Prior replacements like Crystal and Medusa made some sort of sense and even Power Man's brief stint with the team was not at the expense of losing another character.
But what Byrne smartly did was almost immediately throw the team into a situation in which She-Hulk's status as an outsider was immediately apparent and part of a specific character point. He made it very clear that she was NOT a true replacement for the Thing, and her superficial value to the team (physical strength) did not automatically qualify her as Thing's ready-made stand-in. Several of her early stories in the book followed in which she was seen acclimating herself to her new home and her new team.
Some years later, after Mr. Fantastic "died", he was replaced by Ant-Man. (The Scott Lang version, not Hank Pym.) He was another character that was largely a blank slate, but did have the advantage of replacing a hole that was created several issues before he arrived instead of one that was generated by his arrival.
But he never really felt right for the book. That was, in part, because he was never really integrated into the team. In the story, he was cited as being hired for his scientific knowledge and, in that sense, he was filling the Mr. Fantastic's superficial role as resident smart guy. But where I think writer Tom DeFalco dropped the ball here was that he didn't spend any time integrating Ant-Man into the team. They shot from one adventure to another with little down-time, and much of the characterization efforts went into the Invisible Woman and her dealing with losing her husband.
I actually rather enjoyed the DeFalco stories for the most part. He wrote them in a fly-by-the-seat-of-your-pants manner, and readers frequently didn't see where a story might be headed. I found this interesting in that it was different and didn't follow typical plots that were excessively predictable. But, at the same time, it also meant that the overall structure of the title as a whole suffered a bit from a lack of pre-planning. Where Byrne knew where he was going with the characters and often planted plot seeds months, if not years, in advance, DeFalco made some dramatic turns, sometimes mid-story. (I recall artist Paul Ryan once noting that Lyja's pregnancy was originally intended to result in a human/Skrull child, and he was really surprised when the script came in showing that it turned out to be an implanted monster.)
So when examining the integrations of She-Hulk and Ant-Man into the Fantastic Four, it's actually difficult to make direct comparisons. The two were treated so very differently that there almost is no comparison. But, given She-Hulk's and Ant-Man's comic book careers since then, I think it's fair to say that Byrne did a better job endearing his replacement member into the hearts of fans.