Keeping in mind that I do not have a particularly sensitive digestive system and I have always been able to eat just about anything without any issue....
I was vegetarian for probably about 8 years (late childhood through mid teenage years) and when I went back to eating meat I just went straight back to eating it. I still didn't eat a lot of it regularly because my parents were both vegetarian at the time so I didn't eat it at home but I could down a hamburger at a restaurant with no trouble at all. So the meat never gave me trouble.
On the other hand when I ended my first bout of primal eating I tore into a shit ton of food that contained grains (it was thanksgiving and my dad makes awesome rolls and there was some other stuff too) and other than feeling too full because I ate 3 times as much food as I usually do I was fine.
About the only thing I notice about grains is that when I eat them I fart a bit and the farts actually stink. But its not a big issue in terms of discomfort or anything. Just the occasional fart.
So I would say that like everything diet related it really depends on each individual and how you react to different foods.