Are gambling casinos/crypto-games have higher % returns than poloniex and also have same risks as poloniex?
Casino has higher returns on long term, on short term you can have some losses, but it's very rarely to happen, depends the gamblers luck.
The risk on long term is only if the casino scam everyone and run away with investors money, because the house always have profit on long term (that is shared with investors).
That is not so rare, i have seen my funds many times on negative profit after investing into cryptocurrency casino, but you are right about the first part- it mostly was on short term.
Poloniex has pretty decent return of lending rates, you can make up a good profit with enough funds, but you shouldn't compare it to investing into casino bankroll.
Lending business is something where you can be sure that the profit is going to be made, and with investing into bankroll of gambling website you cannot know, if it will bring profits from you, or the site will lose ( and you too).
The second option is mostly about how many players are in there, because if you can see many players doing bets, then you will probably get a really nice gains on long term.