It’s likely that the memcached install is so large that the underlying instances themselves are failing. When you have hundreds or thousands of instances, failures in the instances themselves become pretty regular.
If they start "going bad", something is wrong. That's a signal I wouldn't want to ignore.
It has happened - once an HBA in a storage node was causing occasional corruption, another time due to a communication failure people were building things with the wrong version of something which had a memory leak and would eventually summon the OOM killer. There have been other issues.
"Have you tried turning it off and back on again" is still a terrible system management strategy.
But given the number of people I've heard using "we're on AWS, out of my control" as an excuse, this appears to be an unofficial service they offer.
It's a subtle difference. I think many operators get used to node failures being extremely common when they don't necessarily have to be. I suspect the note on "if they come back on their own ensure they're flushed" meaning they have something unusual causing ephemeral failures. If that's just "cloud networking" there isn't much they can do but it's almost always fixable.
And exactly how rare do you believe this to be?
In my experience, node failures at scale of hundreds to thousands of nodes are monthly to weekly, if not daily. Generally speaking, stability is a normal distribution. Young, new instances experience similar failure rates as old instances. If you have any sort of maximum node lifetime (for example, a week) or scale dynamically on a daily basis then you'll see a lot of failures.
But that generally mirrors my experience that automatic failover for stable software tends to cause more issues than it solves. A good (i.e. redundant hardware and software) Postgresql server is also so unlikely to fail that wrong detection and cascading issues from automatic failover are more likely than its actual benefits.
I'd argue that stable systems are actually worse for operational stability as you become complacent and comfortable and when shit hits the fan you are unprepared.