The SXmini M Class has not been without issues for me. The S Class on the other hand, has been a solid performer since I purchased it when they were first made available.
Lets' see.. So far with the M Class:
E-liquid leaking past the 510 gasket into the battery tube, and inside of the mod. e-liquid was actually coming out of the C frame seams. Kind of my fault for using an RDA with a BF'er pin, but it still should not have gotten past the gasket IMO, and what if one wanted to use an open pin atty, or one of the new Ni200 cartos?
Peeling paint issue. (Resolved by Varitube with a replacement unit)
Replacement unit had rattling +/- buttons, and the battery charging indicator "lightening bolt" became stuck, no longer showing the bar. Varitube resolved the rattling +/- button issue for me, and did not see the charging indicator issue.. Must have worked again when returned, and for a couple of hours after I received it back. Had to send it back again for an entire board replacement when the issue reappeared (under warranty). I should note that I never used the USB port for charging, so the stuck "lightening bolt" wasn't due to that.. It just appeared, and stuck there..
Unit again has peeling paint issue, and also a top cap which either is not totally flat, or the 510 threading is off, causing a noticeable atty lean (especially when taller tanks are installed)..
Don't get me wrong, I love the SX350J board, and how well it works using temp-protection, but to say someone just has "bad luck", or "user error", just isn't always the case.

I've seen almost all of the same issues I've experienced reported from others at the SXmini Brotherhood FB group, so I know they are not isolated. Not common perhaps, but happening nonetheless.