I had a 2005 with an 8.1 and an allison. the only engine/drive train issues I had were either 1: broken serp belt (Hey that happens on all engines) and 2 "Externally caused" problems.. By that I mean the problem was caused by something OTHER than design or engine failure.. Like a road hazard or a rodent. and that too can happen to any engine.
I've heard of Ford having design issues on some of the V-10's (not sure which model years)
Now when it comes to who makes the best engine. Ford or Chevy.. That's dang near a religious argument or if you like a political one (You get a lot of folks who yell one or the other)
But if you ask who makes the best transmoission.... ALLISON ALLISON ALLISON (wispered.. Another make) ALLISON.
Even Ford says ALLISON. (Their latest tranny they advertised as "The answer to Allison" thus admitting Allison was the best).