A lot like Regexes. The first n attempts at a regex always fail, and fail for human reasons - I didn't spot that the pattern I'm matching on actually appears earlier in the writing, the pattern has a typo but I spelled it correctly, the pattern used a unicode character that looked like an apostrophe, etc.

And Vim composability suffers in exactly the same way. "Delete back eight words" works on paper, but eight words isn't a thing you can glance at, it means stopping and counting - especially not if it goes back over a linewrap. And does what about the word boundaries? If the cursor is in the word, at the end, at the start but still has to cross the word boundary to get to the previous word?

Combined with that, the way Vim commands have instant and unbounded effects means that the wrong command or a misspress of a key, and suddenly there's a huge change to the text and you don't know what happened. This leads to the effect where even if a change looks right, if it was a big change ("add a comma to every line just before the last word") needs a lot of careful verification to make sure it worked as intended.

In real world use, almost everything I try to do in Vim outside the everyday patterns of habit, takes enormously longer, works less well, is more fiddly and stressful, than 'just' doing it by hand or in Python.