I disagree. I used to be an AIX administrator for well over a decade, got certified in basically everything related to AIX being on the side hard core FreeBSD user (still have some servers). In my opinion AIX is indeed different, but rock solid and fun to work with.
I've been since AIX v3. Moving to linux 10 years ago was like coming out of the stone ages.
But it's the same for every language. Something that's a stupid hard problem that takes a huge effort to solve is suddenly a one-liner in the next release.
AIX pioneered technologies, like logical volume management, journaled file systems, etc. way before they were available in other Unixes. LVM existed for Solaris, but was an expensive third party add-on (Veritas Volume Manager, etc.)
Command options you take for granted aren't there in AIX.
Search on Unix stack exchange and there's always a more complicated "here's how you do it in AIX."