How so? Signal supports Android and has a cross-platform desktop app (that supports not only macOS, but also Linux and Windows), while iMessage does neither. Signal also has perfect forward secrecy, while iMessage does not.[1] Signal even allows everyone to create and publish sticker packs free of charge,[2] while iMessage requires a $99/year Apple Developer subscription to do the same.[3] I suppose Signal doesn't let you create Animoji or Memoji, but that's not essential to me.