In such a TL chances are the Burgundian Netherlands stay intact, except for parts annexed by France. So no Northern or Southern Netherlands, just The Netherlands, no Belgium ITTL, but TTL The Netherlands might be more like OTL 'Belgium'.
A 'successful' Spanish rule might succeed in completing the Counter Reformation. Don't forget that IOTL Austria at one point had many Lutheran subjects, but eventually the majority was converted back to Catholicism.
Some of Protestants will flee to other countries, so some of the Protestant nation can profit from.
Keeping the Low Countries united is positive, but it comes at a price. It would probably screw the Protestants, OTOH in the Northern Netherlands Catholics won't be relegated to second rate citizens/subjects.