Last I heard Threema was kind of third-place option behind Signal and WhatsApp, at least in terms of use in English-speaking countries. I had the impression it was significantly more popular in Germany, but I don't know anyone in my circles using it.

On the upside, I like that they have a business model ("Threema Work") and features that can make it viable for business use, though being European I don't know if they've bothered with anything related to HIPAA and I wouldn't expect them to support BAAs if required.