I need to make a decision soon on what wheels to get, and I really want Corvette wheels. But, that means 2 inch spacer up front, and nothing in back because I have a 4th gen rear. So, are spacers/ adapters that bolt to the hub, then bolt to the wheel really as bad as people say?

 

I have a set both on the rear of wife's 91 camaro and on rear of my 04 mustang, never had an issue. Can't tell they are there besides the wheels being flush with fenders now.

 

The only down side is weight (minimal) and visual (personal preference). As long as you are just correcting a "wrong offset" and not functionally moving the wheel further out, there is no load difference...
Hub - no different forces than if a stock wheel is bolted on.
Spacer/adapter - no different force the the Corvette hub would see.

(Technically there may be differences due to larger wheel/tire. But, practically speaking, those are insignificant to this argument.)