So I haven't had an issue yet and having the QR code is definitely sufficient in France (and always attempted to check when appropriate in my personal experienc

So I haven't had an issue yet and having the QR code is definitely sufficient in France (and always attempted to check when appropriate in my personal experience, no ID check though as that was deemed illegal). There is however a prevalent issue with QR code scanning that I noticed twice personally (and friends reported the same experiences at other places), which is that most restaurants use a super cheap phone/tablet with a terrible camera that usually is unable to focus on a smartphone screen correctly. Once, the person was honest about it and explained they kept having issues with it, and I tried to help him focus correctly his device (QR code size + luminosity seems to be the issue from what I saw). The second one tried a couple of time, didn't manage to focus and told me "all ok" despite it's device not having scanned me (I could see the screen). If I had to travel though, I'd definitely take the paper certificate on top of it, as, at least here in France, that's considered for better of for worse the source of truth.