It does happen, although it’s rare. Most of the big name (BT Sky Virgin) boxes are made by major PVR players such as Humax.
I must have installed 2,000 PVR’s of various makes since they came out. The only really bad ones were the TVonics ones that were sold during the digital switchover in 2009.
Even the manufacturer couldn’t fix those.
And the first LG DVD/HDD recorders became increasingly glitchy as they got older.
On the whole l have found Sky, Virgin and BT boxes to be very reliable and most issues are caused by installation, signal feed (poor signal or in the case of terrestrial, water in the down lead) or telephone line connections.
Heavy rain caused an increase in phone calls “my Sky has gone off” but the new dishes/LNB’s are better and don’t drop out so easily.
Oh and customer misuse, although it’s always “the box” that’s defective.