have you played in this kind of underground gambling den before?
No, because underground gambling has no regulations and you will get scammed 100% of the time. And maybe worse, robbed or kidnapped.
Underground gambling is glorified in movies and shows but the reality is you are surrounding yourself with shady figures who are trying to get your money from you while making it look like you are indulging yourself in regular gambling activities.
It is best to stick to regulated, legal casinos.
You are right about this, I don't see any reason why someone that's decent would go to an underground casino knowing fully well that a lot of illegal things can go on there based on the fact that it's underground. They use that mediums to also run drugs and other unethical things that are considered to be criminal offences. They are not following any rules or regulations, this is what makes it dangerous