I also had a serious problem with AWS Managed ES. In my case, some of the heap didn't free on every garbage collect which would eventually result in cluster failure. This was likely a JVM misconfiguration and was most easily observed by viewing a shrinking sawtooth pattern on the memory graphs. This resulted in a multi day marathon of sleeplessness keeping the cluster alive by continually rolling it every few hours (We initially assumed we had done something wrong and investigated ourselves first... eventually I shunted traffic to both my own cluster and the managed cluster... my cluster did free heap as expected and we successfully switched over without downtime but wow was it ever hairy during high traffic periods).

We saved a bunch of money and gained performance by using our own cluster. That cluster hasn't gone down since... years later.

It's very difficult to debug these problems when you don't have direct access to elastic search's configuration... what would normally take minutes to verify can take hours to isolate.