The risk is not turning all solar installations "on maximum". That happens nearly every summer day between 1 and 2pm. Automatic shutoff when the grid voltage is rising can be disabled, but more than 9 out of 10 consumer solar installations in the Netherlands deliver their maximum output on such a day for most of the summer, not running into the maximum voltage protections.

The big risk is turning them all off at the same time, while under maximum load. That will cause a brown-out that no other power generator can pick up that quickly. If the grid frequency drops far enough big parts of the grid will disconnect and cause blackouts to industry or whole areas.

It will take a lot of time to recover from that situation. Especially if it's done to the neighbouring grids as well so they can't step in to pick up some of the load.