Other than that, SimpleX Chat has "relays" to store messages just like Nostr does. If you are not choosing your own relays to use SimpleX, it's not that different from Signal, where everything routes through the company's servers. 

And if you choose a new relay, make sure they are not tracking you. They can see a lot of metadata. 

This was one of the reasons that led me to push for a better standard for Nostr GiftWrapped DMs that minimizes (but doesnt completely solve) the power your relays have over you.