That's pretty important. When I needed a mortage last year, I wrote a small mortgage app (in Swift). While it's useful if you always need to enter the same set

That's pretty important. When I needed a mortage last year, I wrote a small mortgage app (in Swift). While it's useful if you always need to enter the same set of numbers, a more flexible app would come with all kinds of UX problems: do I make it fast or do I guide the user? Do I try to cram all time based finances in the same interface, or are there different pages? Etc. And all of a sudden, it has become too complex for lay users, and too unwieldy for frequent users. A simple app suits the former, a calculator that only needs a few presses the latter. If everyone has the same device, even better.