I'm surprised no one has made a QNX compatible/clone? It's reputed to be a very small OS, or at least a small kernel.

I don't really know anything about QNX (like I said, not an expert) so I'm curious: just how general purpose is it? Has it made design decisions based on it's targeted use that are problematic for, say, running untrusted code in a sandbox?

Many car infotainment systems are either QNX or hypervised on top of QNX.

https://blackberry.qnx.com/en/industries/connected-autonomou...

My experience with QNX is quite limited, but the OS is very general. There is a desktop GUI that is powerful enough to serve as part of self hosted software development environment. Generally, the tradeoffs are made to keep the core microkernel small and to give the system reliable guaranteed response times rather than raw performance because it is intended for hard real-time environments.