Quality of wins > Quality of losses, if you're doing a round robin and need your tiebreaker to look that deep. So FrozenStorm won the thread with the first post.
For Swiss, my favorite Strength of Schedule breaker is Average Opponent Win %, but dropping both the highest and lowest value to remove the effect of randomly getting matched with the best player or the worst player in the tournament round 1. The end result is pretty close to Fry's suggestion, but it tends to do a good job being very granular and actually useful as a tiebreak between placements at the top of the bracket.
If the game is played in sets (like, Best of 5), game win % is a barely acceptable tiebreaker. While yes, winning 3-0 is better than winning 3-2, both are winning the set, and that's all that /should/ matter. Maybe somebody has a playstyle that relies on feeling out the opponent first, and those first 2 losses were just scouting data before completely crushing their opponent after the download was complete. It feels bad to punish that kind of player via tiebreakers. But, higher game win % is very highly correlated with higher match win %, so this can still squeak by and give a useful granular tiebreak if head to head or Strength of Schedule don't cut it.
Margin of Victory tiebreakers are completely unacceptable. Average margin of victory is not even kind of correlated with match win%, and making it a tiebreaker completely changes the range of strong strategies for a game, because you're adding a secondary goal. Now, you don't just have to use a strategy that wins matches, you have to use a strategy that wins matches while maximizing your margin of victory. This is especially bad if the design space of the game you're playing uses whatever you're measuring as a resource.
In Yomi, Gloria, Jaina, and Gwen all use HP as a resource. From experience, I can say that all 3 of them tend to end matches on less hp than average than most of the rest of the cast. Because in normal Yomi, not caring about Margin of Victory Yomi, all 3 of them are perfectly happy dropping themselves to very low hp to help get a win.
While in Codex, cards like Dark Pact or Lich's bargain let you sacrifice base hp for current advantage.
I would personally not participate in any Yomi/Codex tournament that planned to use a Margin of Victory tiebreaker.