How good is your vectorized execution engine at dealing with a handful of storage nodes going down for an hour or two? Or figuring out when bit flips have randomly happened? Or at sharing resources with latency-sensitive serving jobs?

"Custom user defined operators" did a lot of heavy lifting at Google over decades.

The set of appropriate use cases is getting smaller, mostly due to sql-ish systems scaling up towards what could actually (and frequently, only) be done with MapReduce/Flume.