I don't know, blame wikipedia.
https://en.wikipedia.org/wiki/NeXTSTEP#Unix
I have no idea how fun it would be to run NeXTSTEP. Although you could probably find out by running the x86 version on a PC, 3.1+ or OpenStep.
I actually did a few experiments in this area recently (OpenStep), and some 15 years ago (NeXTSTEP).
You can install OpenStep for Intel in a virtual machine and give it a spin. You are limited to the very basic graphics hardware support offered by the installation disks, which takes a lot of fun out of using the system (it's greyscale 640x480 VGA). OpenStep and its precursors were intended to be used with a high resolution display, and without it, things become very uncomfortable very quickly.
Both OpenStep and NeXTSTEP are now so old that it is next to impossible to build or install any contemporary software on them. Say, you wanted to play MP3-encoded files or use SSH either as a client or a server application.
Today such software expects a POSIX-ish host, and both OpenStep and NeXTSTEP are far removed from that. These operating systems sit among the branches of the original Unix tree, with distant cousins such as HP-UX, IRIX or SunOS on other branches. Portable Unix software had to account for the minor and major differences between these platforms that shared common ancestors, but were at times so different that it took extra effort to port the code to those platforms.
Because NeXTSTEP is an "exotic" Unix, building client software required the respective project's "configure" script to run and produce useful results, which in turn would permit the code to be built. This step almost always ran into trouble, because either the respective script supposedly supported some version of NeXTSTEP, but that was not the version I was using, or the NeXTSTEP support was by now incomplete and untested. Tinkering with the script sometimes allowed for it to produce some useful build files, but once the compiler and linker got to work, the code failed to compile or link because it had never been ported or tested on NeXTSTEP.
I tried this with OpenSSH and LAME, but both failed in the early stages of the build process. That was in 2001. For OpenSSH one of the obstacles was in building the crypto libraries it needed, and that failed, too. One could, given enough time and patience, port such software properly, but this seems like a really tall order. Even the 'C' compiler is so old that you would have to port a more recent version first.
Long story short: in my opinion you can have a lot more fun with your Amiga than with the NeXTEP or OpenStep platforms today, and that used to be true even 15 years ago
