@alexs, regarding the following from @hugo’s response:
It only asks a manual effort from the users for those users who do wish to categorise their cash withdrawals; those who don’t care don’t have to perform any manual tasks around that.
The lack of being able to categorise parts of cash withdrawals is my biggest frustration with Monzo at the moment. As @hugo mentions, this problem is similar to the problem of not being able to categorise £10 from a £20 Sainsbury’s transaction as Entertainment and £10 as Groceries, but at least that issue has a workaround that isn’t particularly frustrating: We can pay for the “Entertainment” as a separate transaction and categorise that.
The particular problem of not being able to categorise parts of a cash withdrawal does not have a workaround at all. We can write a note that specifies where each bit of the withdrawal went to, but that doesn’t affect our totals for each category, it makes it difficult to see at a glance where all our cash went, and it makes it very difficult to see a running total of cash we haven’t yet categorised/spent.
It’s very unlikely the entirety of a cash withdrawal went on one thing. Even on a night out, one might withdraw £20, but club entry and drinks won’t come to exactly £20; there may be 85p left over or £2.23 left that could be used for a bus ticket a few days or weeks later.
The advice of only withdrawing cash when needed is good, but unfortunately (at least where I’m from “up North”) bus services still require cash for tickets and many shops and bars don’t take cards for less than £5, and “up North” most pints are just a little bit less than £5 
As a software engineer, I appreciate the technical prices to pay (e.g. the fact the product would be more complex), but those technical trade-offs mentioned might be worth paying considering the benefits to users. @hugo mentions that a trade-off with implementing this would be more manual work for the users; however, I’d argue having to write and maintain notes for each of our cash withdrawals whenever we finally spend a bit of it is a lot more work for us users than this feature would be, as would be using a separate budgeting app.