The GI issues tend to be minor. Unpleasant, but not exactly any more debilitating than a lactose intolerant person deciding that they really really wanted that extra large milkshake. Some people have it worse - but those, to my understanding, very much are in the minority.

Tirzepatide also has significantly fewer GI issues.

Muscle mass loss happens in any sort of weight loss where you don't eat enough protein and get enough exercise. There's no current evidence that when you control for calorie deficit, diet macros, level of exercise, bmr, etc., that people lose more muscle mass on GLP1 agonists to my knowledge.