There's good and bag in anything. Bag training included.
Yes. It can give you good feeedback. And yes, it can improve your stamina depending on the way you practise. Let us be reminded though about the purpose of using any kind of equipment.
It's to get your Karate to the next level.
This means that we have to use our brain and use it to our benefit.
You can go all out on a bag - but please... only do that if you want to blow off some steam. It is NOT going to help your skill level mature.