My M8 which I use with Marshmallow and patched in audio firmware blobs from the HK edition is just fine for me. However, the battery is extremely bad now. Turns off at 60%. All the hard power kills are bad for it, I've had numerous incidents where all settings were wiped or some APKs that needed to re-download. Sometimes when it comes back up I notice the screen looks a little better, other times when it's still very low on power even tho it is re-charging when I pull down the notification shade I can see the background flickering. Sometimes very noticably, sometimes subtly. It gets better the more power there is in the device until it is stable until the next hard outage. So there is a definite role power is playing with dithering or flicker in the background.
I haven't decided yet whether I am going to disassemble it and replace the battery, or try a new device. Pretty much everything on the market seems like "same old stuff" to me, so I'm feeling cheap.
I know if I get another device it has to be Android, if nothing else for the Live Transcribe app which I really need.