Really, sweetners like Splenda, stevia, truvia, (sugar substitutes) should cause very little problems. It's the sugar-alcohols that cause most of the GI problems. These are things like sorbitol, mannitol, isomalt, lactitol, etc. Most of these are found in things like sugar-free candy, syrups, and stuff like that.
As mentioned before, if you are having a problem with some something that has a sugar substitute, there's probably something else in there causing the issue. For Crystal Light, I really have to dilute that stuff down or it is cloyingly sweet for me.