Unless I'm forgetting something which mitigates this, Superman showing up at the last minute and destroying Thanos' ship is the most blatant deus ex machina I recall seeing in a superhero movie, off the top of my head.
For what it's worth, this was just a DIF-level nitpick chosen at random; my 'big' issues aren't characteristics of plot points, but characteristics of the whole plot style of the MCU — and like I said, they're totally-destabilizing issues. It's like a 'magic bean' thing, where I'm inclined to just reject the magic bean. My actual opinion is...
this magic bean is unspeakably stupid and the world is an obvious cheap invention with almost no bearing on the actual lives of the actual audience in the actual world, so how much farther do you want movies to drift into pure thematic contrast, and how much more extreme do you think the stakes-polarization can go, because just this week we killed and unkilled our entire planet, just for funsies — all without really challenging the audience in any way, because this isn't meant to be serious — but, thankfully, the combined cinematic efforts of the entire universe [of virtually-immortal characters] was able to win by the skin of their virtually-immortal teeth, which simultaneously makes this random latest movie literally the most and literally the least most-imporant story about life ever... so, tune in next week, to learn what else can just happen!
...but, of course, the point of a magic bean — and, to be clear, I'm saying it's like I'm rejecting a magic bean, not exactly that I'm doing that — is to just accept the damn thing where it's at; not quibble with its validity. It's not meaningful for me to engage in this movie by saying "first, the very idea of it is vaguely abhorrent to me," so... obviously, I have to set that stuff aside; to do otherwise is what would be ideologically dishonest.
(there are roughly two million fascinating stories about what life would be like if any of this actually happened)
But, yeah: accepting the movie on its own terms — and, first, stipulating that the movie has bizarrely-good plotting (and theming) riddled throughout — it still walks face-first into a wall of ass-ness sometimes; possibly because, at the end of the day, their chosen universe is actually just unmanagable if they like their leads to be non-gods and their supporting characters to be otherwise. The fact of the matter is, they've been chasing the dragon of 'stakes' for so long that at this point they're now dragging the stakes of their movies out of the realm of their characters' efficacy — even when they 'team up'! — which requires 'side' characters [with god-powers — with actual god-powers; sorry Thor] to show up and resolve problems for the rest of the characters.
It was clunky.
(But she did say they wouldn't see her again until the end of the movie, so, nevermind. Screenwriting is easy.)