Outside a few open source options like zephyr (and FreeRTOS to a lesser extent), RTOSes typically have a worse developer experience than even the shittiest vendor kernel trees. At least Linux doesn't dictate your choice of dev tools like most of the commercial RTOSes do and the libc usually works.
The opposite. Zephyr is one of the more ergonomic RTOSes. Compare that experience to what's offered by VxWorks or Green Hills, which feel like taking a time machine back to the early 2000s in terms of productivity and features.
