It is in a sense something of a research kernel. The reason to not use Linux is that it’s 30 years old and crufted up with things like multiple users, processes

It is in a sense something of a research kernel. The reason to not use Linux is that it’s 30 years old and crufted up with things like multiple users, processes running as the user that started them, and POSIX. Linux also doesn’t have a stable kernel interface for drivers, which is probably a huge pain when dealing with supporting a device for many years. Either you don’t update the OS on the devices, or you have to modify all of your drivers every time you update the OS.