I did my own budgeting in a spreadsheet for awhile, and those big lump purchases were definitely something that's hard to model.

I think it's pretty easy to create a budgeting product for the 90% case but those odd lumpy purchases and split payments are something everyone wants to deal with differently.

For example, my girlfriend and roommates often split purchases by one person buying, and the other person adding an entry on Splitwise.

That Splitwise entry isn't a bank transaction until maybe weeks later when I get a Venmo payment from a roommate covering several Splitwise requests, and for my budget I want to match up the $100 charge at the grocery store with the $50 Splitwise request with the $250 Venmo payment that was only partially for the $50 owed on groceries... not an easy problem to solve in a general way that makes lots of users happy.