I've never ever ran into a situation where holding the power button for ~15 seconds didn't reset the phone. This stuff runs at a _way_ lower level than the software you can mess with.
Non-removable batteries are still stupid though.
Non-removable batteries are still stupid though.
However, I do know of some slightly older HTC devices that would have the EEPROM straight-up completely die, leaving the device permanently trapped in a "hard-brick" state unless someone felt like doing some BGA soldering.
Modern phones do have a lot of stuff running under the hood that makes it a lot harder to mess things up, but once you go off the beaten path things become much less certain.
You can definitely mess it up really bad, but I don't know if you can get to a point where there's no reset path. Correct me if I'm wrong though.