That's how encrypted messagers work. Anyone can use then for any purpose. It is actually very reassuring that it has become so popular. Just to be clear I am not endorsing Neo Nazis.
Also I like how Simplex Chat doesn't have user facing identifiers. (People can't DM you)
The only concern I have is the public group chats but because I don't use that feature it isn't a big deal for me personally.
This is false. However, you must generate an "identifier / group / channel" for them and share that link out-of-band to them." Basically it means nobody can slide into your DMs unless you yourself consent to it and forge a connection with them to do so. It does offer a way to invite other users to chat; but the other user must consent as well...which makes it far safer usually.
In other words, it's the same effect as when you make separate identities to share with different contacts on any messaging service. SimpleX has adopted that as the normal way to operate.
They certainly make it easier to do so; by making it a switch you can toggle; which allows you to generate an identity; or choose not to and roll with the identity they've already seen.