Using H2H as the first tiebreaker in the group stage is flawed when each team only plays one game against every other team in the group. The NFL does this as well for conference seeding among teams in different divisions - and they're wrong for it. For it to be fair, the group stage would have to be 8 games. Where each team has a home game against every other team. Otherwise, differential should be the first tiebreaker.
Using H2H as the first tiebreaker in the group stage is flawed when each team only plays one game against every other team in the group. The NFL does this as well for conference seeding among teams in different divisions - and they're wrong for it. For it to be fair, the group stage would have to be 8 games. Where each team has a home game against every other team. Otherwise, differential should be the first tiebreaker.