HP-UX 11 was one of the UNIXes I worked on (1999 - 2002, 2005), and the only issue I had, at least during the first employer was their ongoing transition to 64 bits, and the C compiler we had available (aC) was a mix of K&R C with some ISO/ANSI C compliance.

I don't recall any issues with the ISA, and we really liked using its early container capabilities (HP Vault).