It is similar to all RT problems. Even when you have an RTOS, your problem is to write a program that can use as small processor as possible while still guaranteeing worst-case performance. The seldom happening worst-case scenarios often require you to buy a processor where most of the time is wasted doing nothing.

Having fixed time slices just means that every time slice has a hard wost-case time to get the work done. Using a generic RTOS, background tasks can play with spare processing capacity but hard-to-test and hard-to-prove peak loads means you may have to select a processor that has way more time available for background tasks than you have a need for.