I am not sure how much worse their failure rate is versus the norm, but what I think makes it more of an issue is that it's more cumbersome to deal with the issue since they want customers to send the entire unit back, and those are very heavy subs. It's the kind of service you would expect from a large corporation rather than the more personable service you get from a small business. The good news is that Monoprice does honor their warranties without giving customers flack.
From experience of all available sources, owners forums and such, the amp failures are above normal rates. I've had a fail rate of two from two within warranty, That said, I got mine at about 50% off, they refunded one and sent a replacement replaced the other, leaving me with two dead subs and a brand new one.
There was no way to return the dead subs, so I'm going to revive the two failed subs with an external amp.
I've no issues with Monoprice CS, other than they are a big company and subs aren't their bread and butter business, so they don't know what a faff it is to deal with 60+KG subs and the logistics of returns.
Even with the knowledge of possible amp issues, 59% off an already OK retail price is really tempting.
I've invested in an external amp to power my two dead m-15 subs.
Replacement amps weren't an option for me, and the talk of them sending replacement amps for failed ones has been off and on. There's no consistency in their resolution. Nor a solution to the reason for failure, The dreaded 'Christmas Light' red/green protection mode trip.