Many high ranked users comment that in 2018 most bounty campaigns didn't pay rewards. Some of them turn in to a scam, other postpone payments, or just fail to reach softcap. I think that many users give up, because it can be just losing time.
Not just for the fact that users reiterated that point,but it was what actually happened all through out 2018,it was not the best year for ICOs I must be honest with you,more failed projects and just a handful of legit ones
It's a grave risk nowadays to invest in ICOs and if you do so,its best to do a complete research to at least be on a safer side.
The only statistics we can really provide about bounties,is for one to be careful and also learn to be reserved when investing in them as they are mostly scams