The biggest security risk with being on someone else's mobile network account in the US is that someone else has control of your phone number.

These days, access to your phone number basically constitutes verification and authorization from you for many things, including transfers of money.

I control the phone lines for myself, my wife, my mom, one of my cousins, and my sister. But I would not give someone other than my wife control of mine or my wife's phone number, no matter how much I trust them.

>If everyone gets along, there's not a whole lot of downside here.

Everyone always gets along, until they do not.