This vlogger does a great job of visiting five bouillons and showing how well you can eat for not much in beautiful surroundings (except for the Pigalle location)
While the Pigalle bouillon is a plain room unless you are lucky enough to snag a table on the terrace, the food is IMHO slightly better than that at the Chartier Boullons. All are of course industrial food.
Yes, they can be cheap and many Instagrammers love to promote the Bouillons. It's definitely an experience, but I'm not sure I would agree you can it well at Bouillons. There are too many wonderful restaurants at all price points to settle for mediocre.
The food is adequate. It is 'airplane food' i.e. industrial reheated meals but some of it is decent enough. Not the best lamb stew in Paris but decent lamb stew; not the best choucroute garnie -- but recognizable; pretty good desserts. Adequate wine. Cheap, convivial and FWIW Paris is full of terrible restaurants with industrial food for 3 times the price. You rarely get a really good meal in Paris without research and without choosing a real restaurant with limited menu and usually a lot larger price than the places with mediocre food.