I don't use SimpleX, but it's hard to argue against a well developed open source privacy focused messaging app. There are a million "privacy-focused" messengers out there with various flaws around security or sustainability. Matrix is great but the goals seem a little different. Plus, it wasn't that long ago that Matrix was struggling to find funding.

I't is made for different purposes: Matrix is built around communities and groups (like Discord) whereas SimpleX tries to secure direct messages and info channels (like telegram). It has no identifiers unlike Matrix so you can create a whole new "account" for each conversation. This makes it very easy to talk with strangers. Also they plan implementing a meshed network approach (like Briar) so that you dont have to rely on your own interent connection.