Cheap maybe, but if you really use Raspberry Pi a lot, you will find that Pi isn't that robust after all. I installed 20 of them two years ago, so far at least replaced 4 of them during the time. Can't comment on the recent models though.
The only downtime has been a result of power or Internet outages at home.
Anecdotal, but I now have some 36 different Raspberry Pis at my house, and while I've had two (cheap) MicroSD card failures, I haven't had an issue with any of the Pis themselves.
As long as you use quality 2A power supplies (1A for older Pis) and quality MicroSD cards, I don't think you should worry too much about reliability, and if you have a cluster with redundancy... That's kind of the point ;)
They both had the same MAC address on the NIC, which was not impressive and more importantly have about a .5% (consistent) outbound Ethernet error rate. Not ideal for a service utilizing UDP.
I now use Pi's, which work flawlessly.
Now that I look, there are some receive errors reported by ifconfig on the X2, but I've never investigated them. The X3 is on a different network, and reports zero errors. (I don't know how much data either has transmitted, the counter will have wrapped as it is only 32 bit.)
But, I wouldn't recommend them to anyone not thoroughly familiar with Linux. Compared to the Raspberry Pi, there's not much documentation, and the kernel is pretty old.