You don't understand how rebels work with the new system. Rebels can siege land quickly in non-fortified provinces, but if that land is within a staffed fort's ZOC, then no modifiers are added when they seize the province. They must sieze a fort to be able to enforce changes/modifiers/etc. Rebels will either seize the province they are in (which is worth nothing if in a ZOC) or stop and gun for the nearby fort.
That is why it is pretty good to have at least lvl 1 fort ZOC spaced evenly around a wide empire that covers every province. An example is my Ming game, where I have around 100 provinces at start, so I have 14 forts spaced around strategically which covers all of those 100 provinces with ZOC.
System works fine as-is.
Try playing nations for a change that don't have extremely wealthy provinces and trade. You might understand why people complain about forts when they can't even afford to build a single one, compared to you placing building 14 forts strategically around the map.
The system works fine for wealthy nations and it's not working well for nations with poor provinces and trade.