Pi's are a lot more reliable than you may think...
I have about 600 currently running in the field (for the last ~3 years), and the failure/lockup rate is about 1 system every 3-4 months. That's an MTBF (calculated, of course) of about 200 years. Of course that won't hold up over time, but it's impressive.
Basically all of the failures I've had are attributable to external events - either electrical storms/surges, or people physically shorting out GPIO pins when installing things or doing other service work.
I've designed hardware-based watchdog systems into my future control board that these plug into, but based on the data they're not really needed. But, the extra security is always a good thing.