Hm, accidentally hit the submit button before the reply was fully done. Anyway, I think the story point completed, tickets done, pull requests submitted or even number of commits works somewhat well for measuring developer productivity, as long as nobody is aware of it. The least productive developers I've worked with did tend to have very few commits in comparison to more productive peers, but obviously that'd quickly change if they found out that it's a metric being watched, I see no reason for why story points wouldn't cause similar issues. Fight to get the "easy points", less collaboration etc.