Is it always possible to reboot the phone without removing the battery?
As pointed out by my other answer, and confirmed by the OP for the given device with reference to a blog article on the Z1, most devices with non-removable batteries have other means to "disconnect from power", usually "reset buttons" or "reset holes" to be poked with a pen or similar object. This should remedy in situations where the device freezes in a way not even the power button works. But it still leaves the disadvantage of not being able to "switch" the battery to a backup should you e.g. run out of power, of course.