Ah, I've recently discovered that the on/off switch on my QC35II is actually not a hardware off switch at all. My headphones refused to turn off once, which I believe was triggered by being connected to two devices at once, one of which was trying to use the mic.

The "solution" was to unpair everything and to make sure to only connect to one device at a time. I really wish the switch actually just disconnected the power.

Other than that they've been pretty solid so far.

I have the same model, but mine does behave like an on/off button, being enough to solve that problem you had (no sound because some device other is paired) Maybe it's something about a firmware upgrade? I remember there was one in 2018 that enabled an auto turn-off feature past a certain time with no use. I didn't update mine with that or any of the newer ones, so if I forget to turn them off manually they just keep consuming battery.

A year or so ago I had an issue with my one year old pair of headphones (QC35 II). They wouldn't turn properly on anymore (light goes on, but no sound). Reached out to the support. They asked to do basic troubleshooting. After everything failed, they sent a free replacement. They did ask to send the faulty pair back, however. Can't complain about that.