They are almost always rigged as the organizer has to make some profit. But even if not, there is a huge risk that a person will end up loosing everything. People who gamble are not satisfied with winning once, their greed and addiction of game keeps them gambling. Also the greed of money drives them to rig the outcomes too. Also winning free money makes them do many bad things that a person with hard earned money would not have done. Anyhow as Quran says, there are more problems with gambling than benefits. Also even if it is not rigged, the other issues I mentioned earlier are severe enough. Now you now might say, what if they just play sometime and are well in control of everything. Then this will be the same as some people say, “what is wrong with drinking a little if we do not get drunk” The problem is why do something that has a potential of driving you in trouble in the future.
Allah sent us here for a test and gambling is like cheating in a test, just like drinking is all about forgetting your responsibilities and challenges and feeling relaxed and to be bold to perform acts that a person in control would not dare to do.
I heard about people who used to drink in a very controlled fashion for decades and then they went through a severe challenge in life, like divorce, death or business loss. Since they were already on the path of drinking, the stress of these event made them loose control and turned into alcoholic and then they further destroyed their life. A sane person, in such difficulties would have turned to Allah for help and guidance and for peace of mind.