[Sesse__]

> I wonder if there's a trade-off where an optimizer produces better average query plans but worse outliers.

There is. A (now) classic case is running a nested loop with a sequential scan on the inner side. Works great if the outer side produces few rows as expected, a disaster if there's a misestimation and you have a thousand rows there.

The MSSQL and Postgres planners are different enough that you can't really say it's about one thing, though (MSSQL is one of the few implementations of the Cascades framework in a sort-of hybrid AFAIK, Postgres is straight-up System R).