I worked on a team for a short while that decided to use Cassandra to store a few hundred thousand accounts, despite the data changing only slowly and being an exact match for a relational database. It was one of the worst choices I've ever seen in my career, and the only reason I could find was that people at the time wanted to learn Cassandra.
Sounds somewhat familiar. I worked at a place that replaced a basic LAMP stack with microservices written in Perl and MongoDB as a backing store, for the sole purpose of raising complexity.
And left Mongo on the default settings for the time (speed of return over reliability of saving data), so they ended up with a reporting replica which was MySQL.
Don't think I've seen anything before or since which was architected so exactly backwards.
Think my company burned a year on a solution using Cassandra. I think the problem was 1/3 Cassandra and 2/3rds the people that would pick Cassandra amd Java over PostgreSQL and C# or go.
