I really like event card part 1. If it was the only event card this round I'm not sure anyone would be really disappointed or upset. It allows for both versatility, surprise, and vault stress relief. Great card.
My objections are levied at the other two event cards: Marked and Pure. And my objection is based on the idea that event cards should affect all teams as evenly as possible (and this seemed to be a key principle in all previous event cards).
I think the dangerous part of Marked and Pure is that they is has a multiplier in them, which can really change the round more than any previous event card. There is a reasonable difference between normal decks 3 ups and Gen decks with 9 ups. Those kind of matchups are harder (as they should be), but still winnable. All other event cards added evenly to this difference (if used equally by both sides), but this event card multiplies it. A normal deck now has 3 6 or 12 ups and a Gen deck has 9 18 or 36 ups. That is a significantly different kind of up-hill struggle. If a soldier runs a mono, less than half their deck can be upgraded, but a Gen can have a full upped deck. This doesn't make games totally unwinnable, but it creates a huge disparity even if both teams use it maximally.
Marked or Pure might have made more sense early in war when there are a lot of soldiers and no one is on the brink of elimination, but right now (especially with +0/-30) 6 teams face total elimination this round, including the top 2 teams, which will field 8 basic decks against 7 Gens and an Lt. While I don't anticipate everyone running monos... the event cards should take that possibility into account to remain as fair as possible (I'd hope...). If everyone had 30 card monos to run this round, Earth and Entropy send a potential 96 upgrades against a potential 240 upgrades (-144). Where that difference would normally be 24 against 72 (-48)
I don't think it would be nearly as unbalanced an event card if it added ups instead of multiplying them. If Marked was +6 ups and Pure was +12 ups it would still have significant impact, but in an unbiased way. It still maintains the Gen +6 advantage that's worked well all war and still gives a significant bonus to teams that can run monos.
Which leads to my other objection: that Marked and Pure favors teams based on the contents of their vault. All the other event cards affected everyone pretty evenly regardless of vault contents (reinforcements may have helped certain vault make ups slightly more, but it gave all teams the same basic vault bonus to use as they please). Marked and Pure totally help teams that have specific vault make ups: for every mono you can run, you get a bonus (super bonus if Gen mono)... and if you can't run a mono, too bad? True it was a surprise for everyone so no one was planning to keep a mono or two stowed in the vault, but isn't it pretty unfair to randomly reward people for having certain vault contents?
This problem could also be mitigated some by this kind of card happening earlier in war when teams still have large vaults that (by vault building rules) have a lot of in element cards. But at this point in the war Marked and Pure are really a slap in the face and kick in the nuts, respectively, to teams whose vaults are composed primarily of duos (which almost always run off-element marks so they can use in-element pends) or rainbows (although some can run rainbows with in-element marks). They have no opportunity to benefit from Marked or Pure, and that was decided arbitrarily. The theme of Marked and Pure is cool... but it is much the same as rewarding teams for running rainbow decks this round, which would be quite unfair to any team that doesn't have novas or qts left in their vault.
I would agree with several others who have voiced how cool a lot of the event cards have been this war (including card 1 this round!). Pretty much all the events have added new and interesting dimensions without changing clearly advantaging any specific team. I also understand you are in the crappy position of having handed out this event card and feeling you have to defend it against this vocal minority, and we all just kind of have to deal with it (because there's no real precedent for changing/repealing event cards this war). But I think it brings to light several things that should be handled with extreme caution and a lot of thought when creating an event card in the future, and at what points in the war certain cards become unfair.