We found cassandra to be too finicky and flakey and ended up using mongodb - simple to get going, admin, use and maintain.
Cassandra was a huge pain in the ass, had to read all the source code just to get it running!
Oh my. I look forward to your next "X is not a panacea" article on MongoDB.
Turns out commercial db vendors lie. A lot. Never use a feature until it's been in a couple point releases (y in x.y.z versioning). Upgrade slowly.
Also, Cassandra - and MongoDB when used with clustering - requires writing your data to best fit your queries. This often leads to wildly different schemas than you'd create in a traditional relational database.
You should probably read Kyle Kingsbury's article on MongoDB to have some idea of what you're really getting into: https://aphyr.com/posts/322-call-me-maybe-mongodb-stale-read...