Uber essentually built their own custom document store on top of MySQL. They explain their design and reasons (and why they didn't use Cassandra etc) in this post: https://eng.uber.com/schemaless-part-one/
Okay thanks, that explains the why but doesn't make it sound less terrible.
They essentially built a nosql database on top of MySQL. Forgivable years ago but this was in 2014...