I would be interested to see what settings your pro friend was using. Did he have the manual to hand?
I have to confess I've tried it with an STM lens, not via the EF-M adaptor, and I really found it to work pretty well.
I'll be using mine exlusively with the 22mm STM with AF, and via the EF-M adaptor in MF mode.
The horror stories just didn't tally with my experience so far.
Much the same way as the horror stories about 7D image noise and AF problems don't tally.
I am a RTFM kind of guy. I don;t work that well straight out of bed and so I don't expect complex electronic devices to work straight out the box the way I want.
I am a bit of an odd-bodd in that I don't entirely trust AF for anything, and that I was using an SR-T303 along side my digital gear until around 5 years ago, and that my main work camera doesn;t even have an AF mode, but the M seemed on a par with what I would expect, given segment, price, likely aspirations of user - once I had tinkered. It's not going to replace a 7D, 5D3 or 1DX for speed, but then it isn't meant to.
And not to knock your professional friend with his 400mm f2.8, but I don't think it's the kind of lens Canon designed the M for.