[A/UX] I can't say I've ever used it. But it it's SVR4 compatible, it might actually offer better support for building "current" software on it than NEXTSTEP wound up.
A/UX was a mix of SVR2.2 (with bits of 3 and 4) and BSD versions 4.2 and 4.3. It was also SVID and POSIX compliant, and from version 2 it also supported TCP/IP.
To have a chance of compiling more modern software, you need at least A/UX 2.0.1 (which runs well on the smaller 030 machines.) Sadly, while A/UX 2.0 can be easily found, its update to 2.0.1 is next to impossible to find.
The alternative is A/UX 3.1.1 (the latest release) which is easy to find, and which works well on 040 machines, but it is somewhat sluggish on the 030s. I have it installed on both my Macintosh SE/30 and Quadra 650.
The nice thing about A/UX is that it provided an interface layer with the underlying Macintosh ROMs, and it effectively ran Mac OS (System 6 under A/UX 2 and System 7 under A/UX 3) as a process, so you could run Macintosh applications in the same environment. In fact, A/UX presents itself with the standard Finder, and you see that it's really UNIX only when you explore the menus and find the Command Shell
