There can be some differences it seems. For example, I was at the auto show last year and was comparing the acadia to the ranier, which are supposed to be the same vehicle. Well, apparently the size, rating, etc. of certain parts like AT coolers will differ by model.
Obviously the body is different outside, and while some corporae switchgear is identical, lots of other internal parts are different.
From there, it is all about specifying better or worse materials and components. The GMC may well have slightly better specifications for some parts.
Would the actual engine, AT, etc be different? Likely not. But other stuff, even exhaust, PS system (effects handling and desired driving feel), coolers, radiators, etc. may differ somewhat, and may effect down the line.
Would I expect a different MTBF in a commodity part like an engine between the acadia and traverse, for example, if use is the same. But would the dash crack at the same rate with same UV flux? seat cloth wear the same? Maybe not. Materials may differ and be of different quality levels.