Trouble with digital cameras is the complexity their electronics. It's downright impossible to exhaustively test for all possible things that can go wrong in the manufacturing process of large sensors and processors. As a result, some will slip through the post manufacturing test, even if they are only borderline functional. Most of these slip-throughs will probably fail within a very short time frame, but the problems are with those that function for a while and give up after that.
Many of these fail from 'electro-migration', which means that because of the enormous currents flowing through these chips (sometimes even amps), the atoms that the micron wide wires inside them consist of will actually move. It gets worse under high temperatures, as the atoms move more easily then. This can be prevented by designing for robustness, but that takes quite some engineering effort. Sensors and processors would be considerably more expensive (we're not talking 10% or so here, but factors), and we're all complaining about the cost of digital SLRs as they are now, aren't we?
Still, I'll hazard a guess that the failure rate for dSLRs isn't that much different from their (electro-) mechanical brethren. It's just that with mechanical cameras, even if there's a failure, some parts of them still work and you can sometimes make do with them. So we think of them as more reliable. With digital it's often all or nothing..
