WhatsApp claims to be E2E/not readable by Facebook and to my knowledge, all we have to the contrary is speculation provided you verify the keys on both ends (same as Signal). Facebook might know who you’re messaging but that’s also true for Signal. I’d still 100% trust Signal over WhatsApp given Facebook’s massive conflict of interest, but SMS has been known-bad and collected by the NSA for a decade now. US telecommunications companies also have a terrible reputation for privacy. The only advantage it has over any other platform is portability between providers but even that falls to the side since you can have multiple messaging apps at once.