Has anyone had consistent success in using M42 lenses via adapter on the 5D MkIV?

I have used adapters for both Pentax M42 and Nikon lenses on my Canon EOS 5D Mk III. Some lenses will have mirror interference at infinity or long focus distances, where the rear element moves closer to the image sensor. This is common with all Full Frame cameras, but not a problem with APS-C Crop cameras due to the smaller mirror.

Using LiveView will eliminate this problem. The mirror still needs room to swing up out of the way, so before using LiveView, adjust the focus to a closer distance to move the rear element away from the sensor.

Some people will even go to the trouble of shaving the mirror to get the necessary clearance.

Here is a link to a compatibility list showing 401 different lenses that I think you will find very interesting: Compatibility list of M42 and manual lenses on Canon EOS 5D DSLR

I had some of that gear a while back. On the 5D Mark IV, the Helios 44M-4 worked correctly without sticking; the 44 would snag the mirror at infinity. I seem to recall that it didn't stick on the 6D because the mirror was slightly smaller (or maybe it just didn't snag as often/badly), but I could be remembering wrong.