In hindsight, JSONL would have been much easier to deal with as a developer. But I still don't regret picking CSV -- DB interface is pluggable (so one can use JSONL if needed), and I deliberately wanted to have different formats for data storage (models) and data transfer objects (DTOs) in the API layer, just like with real databases. I agree, CSV is very limited and fragile, but it made data conversion/validation part more explicit.