That serious of engine has proven to be very reliable; however, any thing man made can break. What I'm having a hard time believing is what the dealer stated. That engine has been around long enough to have enough parts available for service. I would call the Hummer assistance center and get a case number, and get the dealer on the ball.
Talking with my service manager yesterday, they replaced two engines so far since they started selling Hummers in June.
One was due to the timing chain jumping gears, the other was due to a missing oil drain plug (thanks Walmart, and this repair was not warranty
).
Both engine took less than three days to arrive from Service Parts.