[yobbo]

> ... if you have a transaction that involves three accounts, that would be split up into two balanced mutations, right?

Three or more "mutations", but these might be grouped together in whatever way you want. For this purpose, the meaning of "account" is up to you define. You might call them "accounting objects" representing subscriptions, contracts, invoices, and so on.

Account balances (per transaction) can only be calculated sequentially in the order of transactions, which becomes a bottle neck at some rate of transactions.