In short, YNAB is a tool for analysis of your spending. Qube is a system (FDIC-insured bank + unlimited virtual accounts + budgeting app) for controlling your spending via the combination of 1) a virtual spending card with a “default zero” balance 2) unlimited single-purpose debit cards/checking accounts.
To further abuse memory management as a metaphor, YNAB is a tool for going over your program trace and saying “Now, why did I segfault with OOM here?”, and Qube is Rust’s borrow checker saying “No can do, Ace. If you want to buy that thing with money from bucket X, you’ll need to move some money into it form bucket Y first”.