Luck of the draw I think. If you don't like it, don't vote for N. Also highly likely that these will end up NPC run duchy. But at the same time small can be beautiful.
I'm wondering about this arbitrary rule about them not crossing biome and I'm curious if this is only a pre-launch condition. Once the great game starts and Casus Beli are flying all over the place, duchy claiming independence, new counties forming etc. Could it be possible, say for a whole duchy to change allegiance to a new monarch and be absorbed into that kingdom?