There are different types of sweetener. Some contain idigestible carbs, some sugar alcohols and some are technically not even sugars. A sweetener like Truvia is high in carbs. 100g of Truvia contains 99g of carbs but, you don't digest them so the effect is zero. Truvia contains stevia plant extract and a sugar alcohol called erythritol which is kind to the digestive system. Consequently, most people find it OK. Most other sugar alcohols give you various gastro intestinal effects and can act as a laxative. Many sweets contain these cheaper sugar alcohols and they will carry a warning not to eat too many. Other sweeteners may be bitter, others give you wind etc etc.
I will add truvia to some things but I don't eat anything that attempts to look like a boiled sweet. They invariably send one running to the lavatory.