My issue with SimpleX is that the company is in the UK, and it's developed in the UK under UK law. https://simplex.chat/transparency/

Considering how fiercely anti-encryption the UK is/has become (because "only child molesters care about encryption!"), this is sadly reason enough for me not to trust it.

Do I believe they have a backdoor in their software? No.

But if the UK passes a law demanding they introduce one...