Uber grew net revenue from 2017Q1 to 2018Q1 by 67% to $2.5 billion, which is unprecedented at that scale, and did it while significantly reducing losses. Uber is either the dominant player, or has a stake in the dominant player, in every rideshare market around the world. A lot of governance and litigation risks have been mitigated or reduced in the last year (CEO leaving, very public board infighting, Waymo lawsuit, etc).

The drama last year caused a depression in valuation, but the financials remained solid and the business has good management, leading to an opportunity for Buffett to acquire a stake at a discount.

Full disclosure: I work at Uber, but the numbers I quoted are public and the opinions are my own.