Hi Robert,
We tried that connector first. As you mentioned, being a native connector made it the perfect candidate. However, it is a bit more immature than we were hoping.
For example, I personally fixed a bug where empty collections would cause it to fail: Fixes parsing for empty arrays by javierholguera · Pull Request #466 · microsoft/kafka-connect-cosmosdb · GitHub
Considering how basic the scenario is, I didn’t feel me with confidence. I thought that MongoDB Connector would have seen more usage, be more polished and, assuming CosmosDB Mongo API lived up to its compatibility promise, a viable alternative.