I use a Blackburn helmet mirror.
Took me a while to position it right and get used to it, but IMO it's actually better than a shoulder check because you can switch your attention between front and rear much more frequently and quickly, without unbalancing the steering. These days, I mainly do a shoulder check as a courtesy to other vehicles.
The viewing angle is narrow, but all the angles are covered by a small sweep of the head. The mirror is not convex so I can easily judge how close other vehicles are. I position it so it sits horizontally just to the right of my right eye’s field of vision (i.e. not above or below), maybe 5 inches away from my right eye. I wear prescription glasses.
I sit fairly upright on a slicked-up MTB – with a roadie “head-down” position, a helmet mirror wouldn’t work because you would only see your own shoulder in it.
I cut off the balljoint/sticky pad and fixed it to my helmet with cable ties – I have no issues with vibration. The balljoint has lots of stiction (good) and is probably usable, but I cut it off because there was no flat area to attach it on my helmet.
I guess it could be bumped out of position, but that hasn’t happened yet in the 3 years I have been using it. The arm/pad is made out of a very strong, “chewy” plastic that is unlikely to just snap with normal use.
I think it’s brilliant, but it IS tricky to position and took me ages to get used to it.