I am regularly participating in Bounty campaigns (most times it's ICO's).
I found that it can be really profitable to do so, but I also found the funniest thing is, that you need to do almost the same research (when picking a project you want to participate in the bounty campaign) as if you would otherwise invest in that ICO. So if you pick the good ones it can definitely be profitable.
However it takes some patience and effort to do that, so it's not really free money...