The last question is salient, and it's possible for OpenNext to break and have to catch up to changes in Next.js, though I believe there's some more direct collaboration. I'd say that's the biggest downside - it's not guaranteed compatibility.
I did a migration recently (comments elsewhere in this post), and I don't recall the specific issue, but I _do_ recall running into at least one scenario where OpenNext had made a decision that impacted - in a way that was visible to me and undesirable - how Next.js functioned. That's not a criticism, there's tradeoffs.
It seems like using serverless containers would meet most of the same objectives so I'm not clear where the delineation is here.
I _think_ you can get scale to zero on Lambda by deploying a docker container, too.