The case against him is that he was only a top pitcher for a few of his 20 years.
The case for him is a great record in later years and went 11-2 in the post-season with 2.23 ERA. Those post-season stats are HOF.
The newer methods of stat evaluation are very favorable for Schilling.
I don't put a lot of weight in post-season stats for one big reason: With wild card teams and multiple playoff rounds, there are simply a lot more post-season opportunities now than there were before the 1990s.