I can’t see them lasting long/getting many customers. I was thinking that it might just about be worth signing up, but they won’t do additional/supplementary cards. There’s no way it would be worth having separate Tandem “memberships” for my wife and me. They will also not consider credit limit increases - you’re stuck with what you got when you got the card originally.
I just don’t see how it adds up to worthwhile, unless you have £85K to stick in the savings account.