I have some old metal fillings and don't get any twinges from them when using NuFace.
Although I only use the NuFace on my lower face, during lockdown when I was overdue my regular Botox appointment I used it for a little while on my forehead. I found that it was less pleasant to use on the forehead. Presumably because there's less padding there. I could feel the current a bit (which I don't on the lower face) so had to add more gel which was then getting into the hair line. You can lower the current (I always have it on maximum) which might have helped if I'd remembered.
It hasn't ever triggered a migraine with me but I can see how it might if used around the eyes by someone susceptible.
On migraines, I went through a stage a few years ago of getting them quite frequently. I'm pretty sure the cause was a type of sweetener I was using (sucralose, marketed as Splenda). Before I'd worked out the cause (and stopped using the sweetener) my GP advised that as soon as I feel a migraine starting to take 3 x 300mg soluable asprin. I don't know if that's a well-known tip but I found it stopped the migraine from developing.