Yeah, I've used HP-UX as well. All the fun of an OS and compiler no one uses, with the added bonus of all the quirks of a CPU that really no one uses. I wonder if the soldiers 1000s of libraries deep in the Oracle link line read HN and have some fun IA-64 porting tales.
If you want a dumpster fire for development work, I'd highly recommend HP-UX. HP-UX's stdio wasn't very std back in 1990s. What I remember is a bunch of syscalls seemingly existing but not actually working despite being OK with the same code on Solaris and Linux.
Somewhere around 2003-4, it appears all development basically consists of security patches and new Itanium hardware enablement, and aCC barely supported anything C++ related; its more like trying to use Borland C++, and GCC was really iffy, although it had the advantage that post PA-RISC, HP did adopt ELF.