It’s generally bad etiquette to send SMS because it forces the other side to potentially have to pay money to reply. And if it’s cross-border then just forget it. Unless you are both on iPhones.
Yeah, but it's not considered bad etiquette to give in to WhatsApp's nagging to send your complete contacts so they can build their fscking social graph, thereby grossly violating EU GDPR in that phone numbers are personally identifiable information? And yes, it's you who's violating that law by handing out PII others shared with you under an implied assumption of confidentiality unless you've written consent by everybody stored in your contacts.
If you use SMS then you are sending not just your metadata, but the content of your messages in cleartext to your provider and the recipient’s, and depending on where you/your recipient lives, potentially the authorities too.
I’m all for living under a privacy rock, but WhatsApp is a much better rock than SMS.
