I have multiple savings accounts on budget but that's still cash rather than investments. I don't like having investment accounts on budget because they're too volatile, but in reality I have several I could liquidate and the money be in my account in less than a week.
I think it's more on the concept of the emergency fund - do people really have all their sink funds funded and three months of savings sat in a bank account? It just seems like a lot - my age of money is 100 days because of all of the sink funds/long term aspirational savings (eg a big holiday in 2026) and that's without the emergency fund!
I love YNAB and I've been using it for over 10 years, but I found I have to hold myself back from creating more and more categories to save for costs way in the future, or for sink funds to avoid having to wam, because I want to get better returns on my money than sitting in a bank account.
(Sorry if this seems like a stealth boast - I started YNAB living paycheck to paycheck and in the position that if one of our salaries came in after the weekend rather than before we would struggle to buy food for the weekend. It's gotten us though some very tough financial spots over 10 years or so, but I now struggle with losing control and I can see that as we get closer to retirement and needing to fund two kids through university (looking at £100k each on fees as I'm not in the UK) we need to be increasing the actually savings funds we have at more than a savings account will allow).