I'd trust McDonald's buns and meat to contain LESS preservatives and other bizarre compounds than a pack of burger bread and patties that I'd pick up at my local grocery store. The main reason is that McDonald's is a well-oiled supply-chain machine with (probably) low storage durations for the various ingredients, whereas consumer grocery products can suffer weeks on a store shelf before purchase. The second reason (also just speculation) is that the intensity of regulatory agencies' scrutiny of McDonald's ingredients (especially with regard to meat) would dwarf that of the small packages I would be able to buy for a barbeque.

I believe in staying away from foods high in lipids in general, but fast food burgers don't scare me. It's fries and pizzas that I abhor.