GP was talking about something else though, the 90:90 rule is related to an extremely common planning optimism fallacy around work required to demo, and work required to productise.
The last part of any semi-difficult project nearly always takes much longer than the officially difficult “main problem” to solve.
It leads to the last 10% of the deliverables costing at least 90% of the total effort for the project (not the planned amount; the total ad calculated after completion, if that ever occurs)
This seems to endlessly surprise people in tech, but also many other semi-professional project domains (home renovations are a classic)
"It’s been obvious even to casual observers like myself for years that Waymo/Google was one of the only groups taking the problem seriously and trying to actually solve it, as opposed to pretending you could add self-driving with just cameras in an over-the-air update (Tesla), or trying to move fast and break things (Uber), or pretending you could gradually improve lane-keeping all the way into autonomous driving (car manufacturers). That’s why it’s working for them. (IIUC, Cruise has pretty much also always been legit?)"