[Edi]

Intentional. The rewards are usually worth it, especially with an eye toward the late game. You just need to marshal enough resources first before taking them on. Hellgate Keep, Myth Drannor, Halruaa, Graycloak Hills, Calimport and the City of Shadows should be the toughest ones. Mithril Hall, Waterdeep, Silverymoon and Cormyr are no slouches either and all of those are useful. Myrloch Vale is probably the most dangerous to invade due to the AI grimoire spell selection. Charm with N9 is brutal.

Lots of VPs = use a scout first and start developing a strategy.