The fundamental problem with composer is it goes and updates everything even when I just want it to deal with a single package. And then, of course, because so many packages promise but can't keep proper semver, things break. Marvelous.

Coupled with arcane version syntax (Drupal's version system predates Composer and it used natural syntax like foo (>=2.1.1) bar (2.x) so all they needed to do is to take the about 30 lines of well commented code, no need to memorize a list of sigils but no), excruciating slowness makes me wish I never heard of it much less need to work with it daily.