What he said.
You'll see many times when the ceiling is OVC 001 and the visibility is 1/2 SM ... perfectly legal to shoot the approach... Just might be a little hairy lol
And RVR, when reported, is always controlling. I've seen it where the RVR was 1700 and the reported vis was 1/2 SM... not legal since the RVR requirement was 1800. It's a pain, but it's always controlling by the regs.
Good luck!!