I agree w/ you that lots of giallo and Italian horror fail to live up to their reps, and first watching TENEBRE on VHS in 1989 (?), the morning after I also first watched SUSPIRIA, I felt a little let down. Once you accept that almost no giallo will ever be satisfying from a narrative perspective, and once you realize it was ALL downhill for Argento after TENEBRE... TENEBRE can be appreciated on its own terms.
Me? I appreciate it most these days for that theme song. That's my single favorite Goblin track. For years I used it as the unofficial theme for the theatre summer camp I run for students ages 11-18. You could dub that track over Argento's TRAUMA or PHANTOM OF THE OPERA and even those films would seem tolerable!