I think I've seen all of the Redis/Valkey issues the author mentioned in production.

* Outages where Valkey had no memory policy, ate all the memory, and then caused write errors to its append-only file. Bonus points for another one where the disk itself was full, and AOF writes failed.

* 500s where Redis was fully expected to be live, running, and populated with data for every user, and no fallback to a slower path.

* Creative uses of sorted sets and other data structures which depended on the sets never being evicted.

Despite the observations from the field, I think it's still hard to recommend memcache ahead of Redis. It can be difficult to architect an app to have a memcache-friendly cache layout.

I'd almost guarantee a large enough team using memcache will find a way to need Redis. And then we're maintaining 2 cache technologies.