Quick answer: Unreal Niagara trail emitter with camera-relative space showing flicker on camera move? Per-particle update lags camera by one frame - use world-space trails for moving cameras.
Player movement; weapon trail; visible discontinuity in trail at each frame.
Use world space
Trail in world space; trail follows world; camera doesn't matter.
Or use Camera Velocity
Trail compensates for camera velocity. More complex; preserves camera-relative.
Test moving camera
Each VFX with camera-relative; check during movement.
“Camera-relative VFX have edge cases. World space is the safe default.”
If your VFX has flicker on movement, the space choice is suspect.