Agreed on this one. Most of the promos actually is for keeping the players in the gambling site or to attract new players and usually those bonuses are quite challenging, if you're gambling for a long time you'll know which bonuses are just for luring people so they'll scam them and which are not.
To what you can analyze yourself regarding the bonus, you can also just see what the community has to say about it, usually - people are fast here to find something that's off the tables and nobody is shy to not to speak up about it.
It's the power of a good community.