Not quite clear from their privacy policy but it looks like you have to allow their servers access to your accounts?
If so, you need to be sure that you trust them and their security. I didn't see at a casual glance where they are based and therefore what might be accepted as standard security/privacy practices. Though they do at least mention GDPR and California privacy laws.
I certainly wouldn't use them with Signal (one of their services).
Also, I think you may be sharing your phone number with them in some circumstances? (happy to be told otherwise if you've used the service). Given the recent large-scale Facebook and other data dumps that include phone numbers, this is generally accepted as being poor security practice, you are recommended to remove phone numbers from all 3rd party services wherever possible.
