We have a person deditated to bringing in changes to our polyrepo. Nothing is considered done until it is in his mainline branches so there is incentive to get things integrated. Nothing goes in until it passes the full test suite, whith he verifies you ran before integrating and then runs again to be sure.

as someone who works on core parts that are lively to break everything I spend half of my time just integrating things and anouther quarter trying to figure out how to make my things either less core or not need changes so often.