Hmm, there’s no reason to transcode from most recent Sony cameras into FCPX. I use Sony Z90 and FX3 and everything imports straight in. Perhaps the transcodes are the issue.
It’s not clear what you mean by “artifacts” because there are different causes. Wait for the Flow transition to render. Personally, I prefer to use MotionVFX mMorphCut. It’s hard to be more specific without details and examples. Without specifics, you’re only going to get general suggestions that may not be applicable.
When describing an issue write so that someone can match equipment and follow steps.