As it has been already said, with RTEMS and other similar real-time operating systems it is easy to predict the worst case time for the handling of any event. This is absolutely necessary when controlling many kinds of hardware devices.

This is completely impossible with standard Linux. Real-time Linux might offer some guarantees, but I am pretty sure that they are rather soft.

Linux is already too big to be easy to understand and audit in order to be certain that you really know which will be the worst case.

With a simple OS like RTEMS, a competent programmer can become an expert in its internals in a month or so, and then it is easy to understand what exactly will happen in any circumstance.

I have used in the past some commercial real-time operating systems, like pSOS and VxWorks, but the free RTEMS has been much better than those, especially because being open-source it allows a much deeper customization than any proprietary solution.