I only ever had that random shutdown and reboot issue on my first HTC Sensation (though I had come close on the other two of that phone that I had; it was a known issue with those devices). First one was replaced due to firmware issues causing issues with the device staying on. Second one was replaced because I broke the pins inside the mini USB plug on the phone. Third one was replaced with my Alcatel because it fell from my standing desk. I had far fewer issues with the HTC G2 and the Alcatel OneTouch than I did with the Sensation in terms of crashes and random reboots. I have yet to experience a total freeze. Actually, I guess you could call what happened to my G2 a total freeze, but that was probably caused by damage from sweat from having it in my pocket while working a labor-intensive job (screen went completely white and the device was unresponsive when I pulled it out to turn it on and use it while on a break during a particularly labor-intensive shift).
I've actually seen random shutdowns more often on iPhones than I have Android devices, though that's probably just the quantity of iPhones I support at work more than anything.
I don't use my phone as my primary alarm, though. That belongs to an actual digital alarm clock that I've had for at least 16 years. My phone is and has always been my backup alarm since I first got one. And I've got a pair of spare alarm clocks stashed away for whenever my current digital one gives out, which may just be never.
Really, you should never depend on your phone to be your primary alarm. The random crashes can happen on any smartphone, and I have definitely seen it on iPhones (not on Windows Phones, but nobody at my work uses them and nobody I know has one). Plus, a phone alarm is nowhere near as disruptive as the alarm on an alarm clock, at least in my personal opinion and experiences.