Cassandra session are quite heavy, We have a large farm that spins up, does stuff and closes down. So thats the first problem. (yes we used kafka to pipeline the data in, and that worked well, but...)
It _used_ to be a very heavy write/read ratio. But as time went on, we needed to read more and more thing concurrently.
Because its "distributed" and basically a glorified token ring system, throughput drops dramatically as load increases.
We are not inserting that much data, just lots and lots of records. We then do a geospatial query later on to pull that data back. postGIS is far better at handling this, compared the datastax graph layer + solr(ie, the full datastax "stack" ).
But honestly, we could have coped with that, if the backups worked. That and shipping code with a 4 year old CVE that could have been easily remedied if they'd bothered to do an automated scan.
Every point release would involved 1-5 days of hard work from me. considering the support cost was > my wage, that stung quite a lot.