This seems to be the biggest hesitation I've heard over and over by far. There absolutely needs to be a good story here for both (a) ad-hoc cross-partition queries and (b) automatically building a datalake without having to know what ETL stands for.

However, this isn't so much different from Cassandra/DynamoDB which have a similar problem. You _can_ query cross-partition, but it's strongly discouraged and will strain any reasonably sized cluster.