I contacted a well known wheel builder and asked for a recommendation for a set of 26" 6 bolt disk wheels that would support my colossal 260 pound carcass. He recommended XT 756 hubs and since I don't know much, I googled them.

Well it seems the hubs in question have a reputation. They are either loved, hated or tolerated.
Here's my impression from the internet:

Posts about hub failures:
Seem to be 6-7 years old or older.
Often come down to lack of maintenance.
Are often 2nd or 3rd hand accounts.

Is it possible that since Shimano has such a huge market share, hub failures when expressed as a percentage are not any higher than other brands?
Is it possible that any design issues have been resolved by now?

I'd really hate to waste money on an inferior hub, but I really don't need to buy a wheel set that costs twice as much as my bike does new.

Any thoughts?

Shimano hubs, road and MTB, have a stellar reputation and it's well deserved. The "internet" reports you mention substantiate their good reliability since they are not at all verified or reasonable. Failures due to lack of maintenance are the fault of the owners, not any inherent weakness in the hubs. Go 50,000 miles without changing the oil in your car and see how "reliable" it is.

Use a cheaper Shimano hub -- say Alivio. You'll never break it or wear it out.

What are you using them for? XT are good hubs but if you are dropping them several feet and beating the crap out of them in the woods, failures are to be expected. Lots of the negative reviews I've read have outright admitted an abusive relationship. Your weight, intended use, and gearing can all influence the longevity of a hub.

If you are buying a new hub and not something that's been sitting around for a while, any known manufacturing flaws have likely been fixed by now. I use one of these as a rear hub on a touring bike but I haven't really pushed its limits beyond mostly smooth pavement.

i have never had xt hubs
but
i have 105 hubs on my road bike
which are one quality level down from xt
and they have survived since 1992
ridden in all weather
had the freehub body replaced three times
once because the original failed
once to convert to 8 speed from seven
and once to convert back to 7 speed

in 1997 or so
I repacked the bearings in the rear hub
but the cones were pitted and I could not immediately find the exact replacements
and so i used some generic aftermarket ones
which worked fine
but did not fit the labyrinth seals
and so you can see the bearings twirling around if you look in the end

and i have ridden on those hubs for another few thousand kms after that
all weather and all terrain
with me never weighing less than 250 lbs
and a second frame just broke using these hubs
and I will soon have them mounted on a third frame

in short
my shimano hubs are the oldest and most reliable piece of equipment on any bicycle i own

I have seen them break, but not more than any other hubs really. The riders I've seen that had problems with them would probably go through almost any hubs just the same.

Granted I don't weigh much but one of my spare rear wheels was an XT 6 bolt that was abandoned my it's owner because it always had problems. After I overhauled it and properly tightened the locknuts it never had any issues.

If they had a problem out of the box it was that the axle locknuts were sometimes loose. I have seen riders go through freehubs, but again it's always that one guy who just destroys stuff, and not even big guys.

Also it's worth noting that it's a very inexpensive hub to repair, the freehub is the most expensive thing on it and it's maybe $40? to replace. It's also serviceable with basic hand tools, open end wrenches, cone wrenches and a large allen wrench. It's not exactly fun to service some high end hubs and very few have the tools to do it properly.

I like 756 hubs, and have a few fronts, and one rear built up, I don't do much maintenance on them, and have never had any issues with them. The only bad thing about them is the rear is a bit heavy, but it is pretty cheap, and going lighter would cost a lot more.