I would say extra long allen wrenches (2.5mm - 6mm) are a recipe for a
twisted wrench... bolts those sizes never yield, and 8-10mm wrenches
torque down no problem.

Never yield? Never's a long time. I've seen plenty break, right at the
base of the head. And not *all* my fault. :-)
Usually though, what I've seen is that either the bolt head or the wrench
strips. Maybe it's the bolt/wrench tolerance, but to me it seems like 5mm
bolts can snap or strip easily, but 8 mm can handle more torque than I can
give. Go figger.

It's the effective force at each facet of the wrench. If you give 1Nm of
torque on a 1m wrench, the flats of a 5mm wrench will experience a little
less than 200N at the wrench/bolt interface, whereas the flats of a 8mm
wrench will experience only a little less than 125N. That's a reduction of
almost 40% with only a 3mm size difference.

I've broken cheap 5s that consist of metal bonded together with a seam
running down the middle... a poor excuse for a real tool. Forged tools are
the only way to go.

What kinds of bolt heads have you snapped, and on what specific part?

--
Phil, Squid-in-Training