If you are using temporal anti-aliasing (which is the engine default) then there are a few cases where flicker will occur.
- As was mentioned - Z fighting if faces are overlapping perfectly or close enough.
- In the case of geometry - if some objects are too thin or become thin at a distance then TemporalAA can cause flickering. For example - fences. There are various workarounds for such cases.
- In the case of textures - if your texture size isn’t by the power of two it won’t have MipMaps. If those aren’t generated, textures with high detail will start flickering at a distance since the engine doesn’t have a lower resolution version of the texture available.
Note: I’m mentioning this because if your roughness map doesn’t have MipMaps, it may cause flickering in the reflections.
Those are the ones I most commonly see.
Since you said your reflections were flickering, you might also want to look at their quality settings. For Screen Space Reflections you can find those in the Post Processing volume. There is a slider, bump it up to 4 for best results.
Hope that helps.