There are select challenges but easily worked around for me. My bank, investment firms, credit cards have no trouble at all. But there are certainly a few that offer no easy workarounds- biggest one for me is Ticketmaster which now offers no way to reach customer service at all before 2-factor login via SMS. All support is behind that wall, so one can't even buy tickets through them with a Mac Studio Ultra or similar without select phones using select services. That is certainly frustrating for a customer who has had this same phone number for 20 years and that is obviously in their records for 20 years of purchases. But oh well, there are other ticket sellers who apparently want the business instead.
On the other hand, my total cellular bill for 5G year round is $25 PER YEAR. And iPads don't get "long in tooth" every 2-4 years. My jump was Mini 2 to 6 after EIGHT years. There's a LOT of money savings in a modest amount of inconvenience on relatively select situations that generally have workarounds.
If it's worth the hassle to you, I suppose, and that certainly is a cheap way to keep in touch. I personally think, however, that the ipad (mini or otherwise) is too big to carry around, hell I think the Max models are too big, but then I don't really consume any content on the go except music. I don't stream or watch video away from home except when travelling, I don't use social media on my iphone, etc. I could use my work-issued SE 2022 for SMS, but I don't want to start tying my work cellphone number to my personal life; I give them my desk number if they want one. Additionally, I have zero clothes that I could fit a mini into, other than my winter wool coat and my backpack.
My cell bill is $19/month, and regardless, money isn't a concern. I keep my iPhones for 4 years or so, and they don't feel long in the tooth. I'm happy to hear that it works for you, though!