It mostly just works. Back when I was actively using it it was IIRC the most stable part of the stack. Only went down when daily quota was exceeded. When it ran out of disk, nothing broke, it showed a message in the ui. When space was added, it just started going again like nothing happened. This was something like 2018?

Splunk is not an ELK stack, and there are no other ELK stacks than Elasticsearch, Logstash and Kibana.