I'm a CPA and I recommend my extremely small businesses clients use a single entry excel system. If they use debit for everything they can download the bank transactions in csv. Then use a deep downlist to categorize the transactions. Then a pivot table to display the year end or quarterly income statement. I usually send them the Excel with the pivot table and dropdown list.

Ahhhh, right, my best-loved “we are drowning and have no idea why, can you look at our 40 xls and few html tables” type of client.

Excel and single-entry in general allows one to ignore questions about their financial structure, make decisions that lead to massive analytic information losses and bury a relatively good business in a deep grave of multiple “idk” levels of knowledge. They only become aware of it when it stops scaling (but competition does, for some reason) or a market goes down for a while (but competition doesn’t care, for some reason). Digging them out is a hard and tearful task.

This sounds fine for a contractor sending out one bill per month, but would you recommend it for a larger entity?

I couldn't fit all of the transactions in a given month into Excel for my business. There are more than 1m rows. We are only an SME.

the problem with being a larger entity is there may be legal requirements around your accounting system.

In many EU countries, if your turnover (total amount of money flowing through your business) is over a certain amount, or if you are incorporated (rather than sole proprietor) you have a legal requirement for double-entry accounting, and in some places a legal requirement to have an external firm do your accounting.

Excel PowerBI would easily handle millions of transactions, but it's arguably a completely different product hidden inside an excel menu.

Yes, but once you are using Power BI you have lost the friendly interface the op described for categorising transactions, ie it is not a simple spreadseet any more.