It never seems to work, and it never seems to work for the reasons outlined in The Innovator's Dilemma. Big companies are held hostage by their customers. A new company needs to a.) find new customers b.) be hungry and c.) have total freedom to configure the market around oneself. None of these exist within the cushy environment of a big company - a.) is contradictory with maintaining Google's brand image, b.) is contradictory with continuing to pay the people involved in these efforts, and c.) is contradictory with Google getting first dibs on the spoils of these efforts.
What's wrong with that ?
There are only two YC companies that have gone public and both of those are still losing money -- PagerDuty and Dropbox and as far as I know, none have reached profitability.
If Google/Alphabet does too many investments which fail shareholders will cry.