A GM 8.1L V8 is probably good for about 200k miles if reasonably cared for (a big IF!), but total failures at much lower miles is not unheard of. Few owners ever admit to any culpability if there is an early life failure, so it's hard to guess why they occur.
I'm talking here about the basic engine, block, pistons, valves and heads. That's what the engineers and techs consider the "engine". Most people, though, consider the entire motor as a unit and include the alternator, starter, water pump, a/c compressor, radiator, fuel pump, etc. If the motor cannot be used to move the vehicle, it's an "engine problem". Many of those additional components have a useful life of around 80k-100k miles, so you can expect to have some issues related to the "engine" starting around that point.
You've probably heard that diesels last longer, e.g. 500k or 1M miles. That's true only for the basic engine internals, though. The same types of extra components are used and have the same 80k-100k service life.