We saw the flaws on SimpleX when building NIP-17. Basically, with SimpleX you have to trust the server you are using (both sides) to not log anything down. And they are upfront about this in their docs.

For instance, their recommendation is to use a different server and different IP address for each contact in your list. But their app just bundles everything as one. So, servers can see a LOT. They have promised to not log things down, but if they want, they can (I run a server for a while to test these tracking capabiltiies out).

Ideally, you should never user their default servers because if they can see both the receiver and the server channels in the same machine they can link a lot of people together and slowly figure out who is who. 

NIP-17's goal was to reduce the metadata leaks to the relay you are using such that you don't need to trust to not track you down. With the help of broadcasting relays, it's virtually impossible for relays, including your own inbox DM relays, to figure out who you are talking to.