When a small, very lightweight UNIX-style programming API needed to be used, QNX was often a favorite.
I sometimes think QNX must have had a really good salesperson in the 'Rust-Belt.' I know I talked to several fans in companies doing that work. I do know of a least one firm that still uses it. An inexpensive x86 can be designed into a custom controller, and the only 'development' is the customer interface to private HW. The development system is a PC or Vmware on an engineer's desk.
After Blackberry bought the company, it's interesting that they seem to be all that is left of BB. But they are still going strong: QNX Neutrino RTOS
Funny, I loved Mach (still do), but it tried to be all things to all people. The QNX guys did not. I also think it help in their success -- by the fact that the QNX folks concentrated on RT, while Mach tried to be the replacement for all of BSD. They both have their place ... I'm typing this on my macOS 13.0.1 (Ventura) M1 system, which is just the current flavor of Mach. As Tru64 hacker, as well as one of the folks that work on Intel Paragon, which was OSF/1, all three are Mach based.
I also did some work with QNX back in the day and, like Larry, was always very impressed. At one time, I did some consulting in the Rust-Belt, and the executive (i.e. -- Havard Business. School types) asked me if "this QNX thing their engineers were using -- after all it was not from Microsoft, IBM or DEC, of course.'' [they had converted/were in the process of converting from DEC PMAX-based controllers running Ultrix to PCs running QNX]. My analysis at the time, for a bunch of ex-Fortran Mech E's, had done extremely well. I told the execs then that this is good stuff; it's going to save them buckets of money as it 'just works' (that was the core SW in the automatic 'sorter' that at the time was being done under contract for Amazon -- I know the CEO of that firm and they sold the same basic system to UPS/FedEx/USPS -- they used to have a very cool movie taken during the testing at FedEx with glasses full of champagne moving at 45 miles through the sorter without spilling -- the PMAX would never have been able to do that).
Frankly, for anyone learning either about microkernels or RT, I would certainly tell them to look at QNX. Neither topic are what we call 'research' projects as much these days, but both have extremely practical applications.