I haven't kept up in Apple-land, but I believe the NeXT stuff is still foundational to their whole range of current OSes. When it was still NeXT, they went from supporting only Motorola processors to Sun's SPARC and Intel's processors, in hopes of finding a bigger market. I understand that OS flexibility enabled Apple to be much more strategic in their processor choices.
Interface Builder was mind-blowing. It's why we have .nib (or is it .xib now) files on macOS as well as all the NS* classes. The layout constraint solver was really slick back then too.
I suspect that 'porting' the operating system, but not really supporting machines of the day didn't help.
I think the reason for modest hardware support was that at the time they were chasing the high-end custom development market that had been so receptive to their workstations. At the time I was working for financial traders. We were buying machines specifically to run our in-house software, so we were happy buying whatever was supported.
Their strategy for broader adoption was OpenStep, which was taking the great NeXT-specific stuff and making it run on Windows NT. It would have been interesting to see where that went, but as you know Apple's acquisition put an end to all that.
Also they only had 250 or so employees. So not a lot of staff to track down hardware compatibility issues, and not a lot of motivation from vendors to help.
That said, they did pretty well.
Some non-GUI bits for "Portable Distributed Objects" servers were apparently available on Alpha.