I'm not familiar with them personally, and don't plan to switch away from Signal anytime soon, but this might help anyone that was looking into Threema :)

Some discussion about it:

• https://www.securemessagingapps.com/
• https://discuss.privacyguides.net/t/threema-instant-messenger/1679

TLDR, it looks good from a technical aspect but being a paid app makes it harder to recommend, and it doesn't do well with the network effect as a result