Quote Originally Posted by Roark
The only problems that I can see with the hub defence strategy are the following:

1. Upsets to unit production (specifically units like cannons, ships etc, that take 3 years. These revert back to zero if the province is removed from your control.)
2. Upsets to building construction (19/20 years completed for a citadel. Suddenly you are attacked and the province removed from your control... BAM!! Back to zero, mate.)
3. Retaking mountainous or river provinces the following turn.
4. Reassigning governorship (more an annoyance than anything)...
Well some common sense is required too when implementing this. Basically the hub system relies on your frontier provs being just frontier provs, while you do most of the buildings and unit production in your hub province. Basically I personally only build my frontier provs up to castle+catapult towers and some economic upgrades. More just ain't viable for them.
About the third point, you're correct with the retaking of river provs and mountaneous provs. That's why it's actually better to have a medium sized army defending these provs, because they're not worth abandoning.