1. You have to use a phone number. This can be a VoIP or Twilio number but the security of phone numbers is sketchy. I would prefer to use a non-phone number identifier as my account identifier (eg: username) AND use TOTP or U2F multi-factor authentication.
2. The app asks for access to your Contacts by default. This is a blatant security violation, IMO.
3. For security-conscious folks like me who will NOT allow Signal to access my contacts, I cannot apply a display name to a contact in the app. This has the side effect that I'm memorizing people's phone numbers again but it also means I frequently forgot with whom I was having a conversation which can lead to accidental disclosure of info, hilarity, or both... but could lead to disaster as well.
4. Give me an option to never, ever display stickers, dammit! If I wanted cutesy crap like that I would use FB Messenger.
https://en.wikipedia.org/wiki/Form_990
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990 is intended as a document For public release; the 2018 is the most recent 990 I was able to find for Signal, if anyone has a more recent one, please link to it.