McDonald's has grown fat from it's success. It doesn't know what struggle tastes like. Perhaps it never knew.
Burger King, the eternal second, the perpetual last born does. If I fall in the mud, Ronald wouldn't help me up and risk getting his hands dirty. But the King? He would jump right in with me for he is not afraid to just put three different kind of meet between a bun and call it a burger.
I'm not gonna lie, I straight up love taco bell. I avoided it for years and years because of the stereotype about it making you sick or being cheap and gross. Then I was out a few years ago and had 2 dollars in change and an empty belly. That shit was good and no stomach upset. It's now my go to fast food, especially considering it's easy as hell to get veggie options.
Also, gas/bloat is not specific to taco bell, it's fucking beans. Eat anything with beans and you're gonna get gassy. Not directed specifically at you, just anyone who needs to hear this.
the stigma that it uses low grade beef, since then taco bell has completely flipped it and uses beef at a grade higher than other fast food restaurants, its just that not everyone knows about it.
the other is people's shit diets. people only get gassy/get the shits at taco bell because taco bell food as you mentioned, has beans, which has fiber. fiber is good for digetive systems and can turn into diarrhea if it wasnt part of the diet before hand. admitting that youre getting the shits at taco bell is essentially an admission that you have a shit diet (not enough dark greens or beans)
getting the items fesca style greatly improves taco bell. im not saying it makes it good but I will take it over standard BK. Requesting fresh from the broiler/fryer greatly improves BK. That being said wendys is good without having to request they make it better and mcdonalds is actually way better than it used to be.
I have a stomach of steel. I only eat fast food like once a week - McDonald's, taco bell, chipotle doesn't phase me but BK absolutely wrecks my gut for 24 hours. I've tried ordering different things but the result is always a shit tsunami. Which is a shame because it's so tasty.