I think the biggest bugaboo I have against them is that pushing the wheels out changes the alignment geometry a bit, as well as steering and handling behavior. It also would have to (because physics) put a little more stress on the wheels, axles, etc - especially under extreme conditions, since you're effectively stretching the axles out a bit (not literally, obviously, but effectively.)
Short/medium term and normal driving, yeah, it likely won't hurt a thing as long as they're decent quality and are installed correctly. Long-term (150k+ miles), or under extreme handling conditions, or under heavy loads? I'm not so sure that such an arrangement would hold up as well as stock.
One nitpick: Yes, dually trucks have fat spacers, but they were engineered for that, so it's not a fair comparison.