When I was working as a CSR, I noticed that IRDs were non-recoverable if the following conditions were met:
IRD has been active for 2+ years for Advanced Products, 1+ for standard
Failure for Code 21 or 17 (and others) in some cases on 22 and 21 models, rarely 23 and 24s
Multiple prior service calls
Refurb units (all of the H23s, HR24s and H25s on my truck are refurb, fyi, and I ran in to a refurb HR44 last week)
IRD will not turn on
I've had a few with noisy fans that the system didn't want returned, guessing it was related to one of the reasons above
Same goes for a few with display issues.
I think I've actually had to go over the equipment return policy less than I've had to explain to customers that they don't have to return their stuff. OMS (the ordering system) and RIO (the accounts manager) don't make as many mistakes on returning hardware as they used to, so you can be fairly comfortable knowing that if a rep says they don't want it back, that's not going to change in a month. I still recommend keeping the stuff, though, and when I do a swap for any reason, I take everything back to the office to RMA, whether it is 2 years or 12 years old.