Flatscreen UI generally doesn’t work in VR – you have to move it “into the scene.”
For UMG / Slate stuff, that’s pretty easy using the widget component; attach it to the camera to position it at some known offset.
When it comes to full-screen effects, the result in VR depends entirely on what the effect is. For example, film grain may be super annoying when it’s different between the eyes, and generally will make the experience worse because “reality” doesn’t have film grain. Meanwhile, a soft vignette may work fine.
This is what I’ve found so far – I’m by no means an expert in this area though.