Stevia isn’t bad in the sense that sugar is bad.
Some say that stevia is bad because it tastes sweet, and therefore it tricks the body into believing that a lot of sugar is being consumed, and hence a lot of energy is available. This may nudge the body toward the wrong behaviour.
I do not know if this reasoning is correct.
The only real problem with stevia I know about is that the maximum daily intake is quite limited. I believe it’s as low as 4 milligrams per kilogram of body weight per day. If you use stevia in only some of your food or drinks, that’s no problem. But if you use stevia for all your sweetening needs, you will easily exceed the maximum daily intake.