Not every country has unlimited talk and text as a widely as others. I know my husband's family uses what's app because they can always hop on their WiFi or a neighbors and talk to family, but they can't always afford to top up their minutes. The social drawback isn't that they'll look at you funny, it's that they might literally not be able to communicate with you.

Add in that some of those families also play hot potato with phones, swapping who has what phone almost weekly, something that follows the login and not the phone starts to make sense. I know there are better alternatives to what's app and don't defend it, but getting them as a whole to change apps so they can all communicate would mean a lot of work and energy I can say they don't have these days.

Probably referring to group chats and sharing media.

My point is you need to put your foot down and say "I won't use WhatsApp. If you want that functionality with me, we can use Signal, but otherwise SMS."

WhatsApp really doesn't have any features that aren't also in Signal, but Signal isn't owned by Facebook and was never a vector for zero-click access to your device (NSO's Pegasus toolkit used WhatsApp calls to get at Android phones, this was involved with Saudi Arabia's execution of Jamal Khashoggi). WhatsApp is simply not trustworthy, and a massive security risk.