Not suggesting you didn't experience these things though of course.
Early '00s for the malloc issue with things that used autoconf. I first ran into this when attempting to build AIDE + deps on AIX. Creating a wrapper around AIX malloc and linking it in was easier than re-working a bunch of autoconf scripts.
I haven't touched AIX since the late mid-00s. My comparisons were to the state of its contemporaries, at the time.
Yes, nothing is perfect, but AIX was the wartiest, and experience from other NIX transferred the least, of the commercial NIXs that I've used (IMO, Solaris was the nicest of the commercial offerings [pre-Oracle]). But, it looks like AIX was liked/loved by quite a few folks per this comment section-- I wish we'd had some of these folks on staff at the job with AIX boxes, so those of us less enthusiastic about the platform wouldn't have had to deal with it.