I think you're misinformed. Uber tells the rider the ride will cost $5. Uber doesn't tell the drivers how much the ride will cost until the trip is finished. Then the fare is calculated based on miles and minutes. If the fare ended up > $5 then Uber will pay for the difference. If the fare ended up < $5 then Uber will take the difference. Seems reasonable to me.

How often would you think Uber undershoots the estimate?

Probably not that often - but it doesn't sound like its risk less for them

well they're losing billion of dollars every quarter so I'd say pretty often.

I meant: how often do they lose the spread?

On the other hand, if the ride ends up paying the driver $6 because the driver has to change routes because of construction/traffic/etc, I believe Uber eats the $1 difference

I didn't gather that from the article; if you are right then it will be apparent in the court case.