EC2 instances are inherently unreliable and that's not a knock on them, that's exactly the contract that you get using them and you're supposed to plan your architecture around the fact that at any moment an EC2 instance could die. We lose about 2-3 EC2 nodes per day (not like our app stops, like Amazon's own instance health goes red) and we couldn't care less.

What percentage of EC2 nodes is that?

Empirically around 0.1%

Setting limits is important, but it always has been. Kubernetes nodes typically don't have a swap so without setting container limits, some critical process can OOM. With swap enabled, memory grows > pathological swapping ensures => caches get dropped making disk performance suck, and all the while your system is shuffling pages between memory and disk. So of course load hits 50+ and the machine turns into a 'black hole'. I've even seen a single VM do that, and cause so much disk IO that it took out the whole hypervisor (which had a single RAID volume)