For what it’s worth I have several, in different anvil configurations and sizes.
All give the correct reading(s) on gauge blocks
SPi seems to work well
M&W feels OK in use, though they make more than one grade.
I’ve an unbranded one in a plain blue box just labelled IP54, which works as well as any of the above.
Lastly I’ve a couple of the very cheap Chinese ones; mine came off eBay for less than £15 delivered.
It’s a bit pot lock with those; they have been discussed on this forum before with different folk having different experiences.
Some looked like the carbide anvils had never been ground, or at least finished very badly.
All 3 of mine are highly polished, appear flat and parallel, and are good enough finish for a gauge block to wring on OK.
These very cheap ones in the plain wooden boxes do have a comparatively short battery life.
This last point leads me to another; The cheapest ones, and the M&W, all have thimbles with metric graduations, so they can still be used with a flat battery.
My SPi has imperial graduations, but the unbranded IP54 one just has a plain rubber/plastic sleeve on the thimble, so is useless with a flat battery.
Bill