You may find it difficult to find a UPS that will last that long without a breakdown. The charging circuits in low-cost <$5000US/kw types tend to be rubbish, so the batteries may last a year or two, but eventually will fail. The "good" ones tend to be large, i.e. on the order of a dual 6-foot rack cabinet, and have redundant battery arrays, allowing them to cycle the batteries being charged while the other battery array is in ready status.

Microcontrollers with properly designed power supplies and thoroughly tested software should have no trouble at all with a constant (24/7) duty cycle. The ones on the Mars rovers have been running for 5+ years. Consider the ones that run the various communication and weather satellites ... and the GPS hardware in orbit.

The key is in thorough testing ... not just "try it to see whether it works", but real testing, in which you subject a batch of what you consider to be fully functional and perfect-in-every-way systems to all the conditions, extreme heat, extreme humidity, extreme cold, chemical pollution, EMI, shock, vibration, for an extended period, typically 1000 hours (nearly six weeks), all the while subjecting the systems to all conceivable combinations of data through whatever data channels it has. You must, of course, exceed all allowable limits on temperature, humidity, shock, vibration, EMI, etc, so you know where the limits are, so you know how the system will respond. The system must be designed such that it responds to an upset in an orderly way.

The crux lies in designing the test procedure so that you don't immediately damage/destroy your sample batch of devices.

RE