Unfortunately, I could only second this opinion and add a few points of mine.
1. Many unexpected behaviours which you would not encounter in any other software - for example when you're blocked by its firewall, it would let you connect on all 7 OSI layers, then kick you out with "Command failed with error 13: 'Not Authenticated' on server xyz:10255'". By the way even Azure Portal itself doesn't know how to handle this behaviour correctly and when not included in the firewall whitelist just blinks like crazy.
2. Many seemingly basic features commonly found on other database systems are for some reason missing here - it doesn't support users as such, just issuing resource tokens[1] (which you must implement yourself). Thus, most CDB users just go straight with the master key even on production.
3. It is not fully (nor at least nearly fully) compatible with any of the advertised APIs - SQL doesn't support joins, MongoDB misses quite a few features and adds a requirement for the shardkey to be part of the query. Which is not supported by Spring Data, so you must do low-level queries for everything but select (think updates field by field with null checks).
4. The tooling is horrible - as it is not compatible with neither mongodb nor traditional sql, you would need to include the "CosmosDB Emulator" in your build in order to have at least some predictability. The problem? It only runs on docker4win and a Windows VM would need to be running by all developers on the team and workers of the CI/CD. As of April 2019
5. It is CRAZY expensive and the pricing is not really transparent - in our case we had 20 MB of data, 4 1000RUs collections and multi-master write in Western and Northern Europe. The monthly bill? 300 EUR even though Azure Portal was displaying 1/6 of that cost - apparently for multi-master you multiply by the number of regions + 1 (2+1 = 3) and for geo-redundancy only by the number of regions (2), thus the 6x higher cost. The result of that was that we were paying several times more to manage 20MB of data than for all the remaining data - hundreds of GB.
Maybe if you really, really need the planetary scale, the master-master replication and the 1. https://docs.microsoft.com/en-us/azure/cosmos-db/secure-acce...