But I will argue that every mission is winnable, by any team, with any given setup, with any amount of xp/threat/money.

I disagree. We played "Sorry about the Mess" this week. Take a look at our campaign sheet (which is already almost updated to after the mission ... just need to add the Agenda Cards I bought):

https://docs.google.com/spreadsheets/d/1b2xKRghuOok9JDLsJjuU9Pv1QTa0ZnHM53vUwhPCxgk/edit?usp=sharing

In the first round, 1 shot from 1 Stormtrooper on Han was enough to deal 3 damage + 2 damage (with Surgical Strike). The Rebels tried to get restorative items from the chest, but didn't have any luck. I saved threat and did not deploy after the 1st and 2nd round. End of 2nd round the elite Trandoshans were deployed. They hung back to flank later. The Rebels opened the door early in the 4th round (first activation actually). I deployed a Nexu, Royal Guards and the E-Web inside the hangar. Han takes his free shot and hides behind the other Rebels. During my activation I activate the trandoshans and deal another 5 (3 + 2 from Surgical Strike) and another 3 (4-1 defense), ending the mission. I used only 4 of my XP cards (Weary Target, Prey Upon Doubt, Surgical Strike and Savage Weaponry) and the Rebels basically were helpless. Even if they would not have opened the door but instead turned back on the elite Trandoshans I could have deployed a sizable force at both deployment Points to wedge them and slowly chip at Han.

Surgical Strike contributed 4 damage on Han this mission over 2 Attacks. This mission doesn't seem winnable late campaign against Subversive Tactics, even if the Rebels know the mission triggers.

We played "Sorry about the mess" last night, and through a minor deployment error (not giving Jyn Odan LOS to the elite probe droid), I was able to hit Han first round with 9 damage (including self-destruct), and proceeded to shoot him with a spawned elite Storm trooper squad in round 3. It was never even close.

Funny enough, I lost "Fly Solo", which also seems like a surefire win to the Imperials, because our luckiest player rolled 4 full dodges, 3 of them in a row, to get Han through a firestorm of shots, and IG-88 failing to hit twice in the same actication by rolling the lowest range numbers possible. It was probably the most lopsided game in terms of dice luck I ve seen so far, but it was great fun nonetheless... and it resulted in my players getting 1100 credits (4x 100 base, 4x100 extra, and luck with the 100 credits chest item) and 2 xp, which after losing all but Under Siege beforehand, is a big step towards balance.

I believe if I had won this game as well, at 5 loss 1 draw (more or less) throughout the campaign, we d have hit the point of no return in terms of balance.

BUT: There are some Agendas and Skills that really exacerbate the problem. For example, the Agenda which forces the rebels to pay to get rid of it is a bad snowballing thing, as well as anything disabling their abilities to utilize expensive bought gear. Also, IMO, Jyn Odan is about the hardest to play (but exceptionally strong) characters in the game, using (or failing to) use her makes or breaks the rebel game in many situations, as that additional attack per round is extremely important when protecting yourself from melee rushes.

You may be overlooking the possibility that the designers anticipated snowballing and allowed for it in the mission structure.

Not all missions are equally hard. Suppose that if you win, you get buffs, but the next mission is harder for you (a "hidden" debuff)... snowballing would not therefore occur.

You need to take a more holistic view of the game design (and allow for the possibility that the game designers may not have been completely incompetent) before making sweeping changes. First you would need to be sure, and not just from a single anecdotal data point, that snowballing is actually a problem, before seeking to fix it. Yes, the winners get more rewards, but the cards they are holding are not the only factor in determining whether they are more likely to win the next game.

Like I said a bit further up in the thread: this is our 2nd campaign. Both of them have snowballed so far (and pretty hard I might add).

I also took a deep look at all campaign missions (the only ones that actually get selected by the outcome of the previous ones) and there is some snowball mitigation, but sometimes it actually seems the opposite.

Currently I think certain combinations do not work well in terms of balance. And the changes I listed earlier actually don't change the balance overall, it's mostly just the metagame.