When playing in a squad, players with an Xbox Series S|X can load out of the mission completely and choose the next mission before Xbox One players have even made it back to the orbiter. These Xbox One players will not have the mission vote shown to them, leaving everyone just waiting around.
Another case of cross-play being an issue is the Xbox One player being the host (the one who invited the other player) and launching into an open world. The Xbox Series S|X player will load in before the server is even ready on the Xbox One, fail to join it, and end up back in their orbiter alone.