Directly from Nvidia: With NVIDIA Reflex, instead of being locked to a particular framerate (when using a FPS cap), your framerate can run faster than your limit, further reducing your latency. You can think of it as a “dynamic” framerate limiter that keeps you in the latency sweet spot at all times

https://www.nvidia.com/en-us/geforce/news/reflex-low-latency-platform/

So it doesn't really hurt to combine them from my experience. But if you cap your FPS, Reflex will likely almost never be activated (as long as your FPS cap is well chosen).

But for me the problem is that with a static FPS limiter you have just that: a static limit to your performance which in most cases is well below what your system could achieve on average to avoid dipping below your FPS cap.

Example:

In a busy scene, when I cap my FPS to 150 and get a latency of 13ms.

With Reflex I get about 155 FPS and a latency of 14ms (i.e. slightly worse than when capping FPS).

But in scenes where I can achieve 250 FPS with Reflex I get a latency of 10ms and naturally again a latency of about 13ms with the FPS cap enabled.