I (Moto X 2014) got a different set of bugs on M. Mostly problems with basic functionality:

- When I get called, half of the time I don't see the name of the person when they are in my address book.

- Music controls (which were already very basic) usually stopped working a certain amount of time after the last restart.

- Music randomly pauses when browsing the web at the same time (as in, I have to open up the music notification card and press 'Play' again).

- E-Mail notifications have been flaky for some reason.

- Active display music controls are worthless again. Google decided to add a 'favorite' button. Motorola just picks the first three buttons from the notification card. Now you can't go to the next track anymore.

Security updates were also still at the 1 November 2015 level. After being stuck for half a year on a buggy 5.0 release before getting 5.1, I know this is going to take months to fix, if ever (Lenovo probably doesn't care about the 2014 anymore).

I am now back to an iPhone after a Nexus 4, Moto X 2013, and Moto X 2014.

Now that you need to bring your own phone to most carriers, there is no reason not to stick with Nexus/pure Android. The updates work there for the most part.

There's also similar quirkiness with the VPN client. Does notifications for random events, and they aren't even consistent.