Rollbit has great reward system indeed but I think their referral system is really bad (at least on the lowest level).
I invited my friend to play, he wagered $165 000 and I got only like $170 commission, when on most other sites that would've been at least 350-400+
I would say there are different systems for each casinos and we can make comparisons but I would say at least you got something where some casinos are there who won't pay you any single penny for it.