For me, the gems that I choose are:
1. Managed by famous BM
2. Bounty funds via escrow
3. The bounty allocation is reasonable, not too big and not too small.
Then just do a little research on the project to be followed.
That's an important point, because we have to make sure the bounty is managed by a popular manager and the bounty payment is escrow status. But don't forget to add that each project must be analyzed for potential because some low reputation projects also use the escrow payment method.
I agree because basically before joining the bounty it would be nice to do in-depth research and analysis,
anyway it's important but sometimes not everyone do that,
there are many bounties and we must be really observant in choosing