Preempt RT does give guarantees in the sense that unbounded latency is a bug and theoretical maximum latency bounds are known (see [0]). It is neither certifiable nor formally proven, but it's good enough for almost anything that isn't safety-critical. For the things that do require functional safety, you can use AGL and other hypervisor architectures that partition the critical and non-critical tasks with a few more changes to your code.
[0] https://bristot.me/demystifying-the-real-time-linux-latency/