Look at last War. The last two event cards killed off two teams almost by themselves. Zeru would probably not have lost against me if he could've used Discords, and I would have won the final two matches if Napalm didn't get a free Firestorm from the event card (and drew it twice in a row), and thus I would've have won War 3 for Team Aether. Luckily, I don't think cards like that will be repeated in critical moments (the last rounds when teams only have 1-2 decks left). At least I really hope they won't.
Being in the lead, many event cards are detrimental because most allow opponents to adapt to the current situation rather than the team in lead continuing with the status quo and the momentum they gained. The more matches you have, the less predictable event cards become, and thus the team in lead is guaranteed to get the most resistance. The matchups are just another example of this where the team that have to play the most matches get the most Lt.'s and Gen.'s.
In the end, event cards that can change things up in a fair way are the best, because although there is always hard work behind success, there's also always luck. More specifically, starting vault matchups. Event cards that let you adapt test your true strategical skill much more than just starting vaults, especially since luck with the latter has a huge impact.