[Kobal2]

Well the thing is, blood nations never have enough slaves (even when they're pulling 400+ a turn) and they're especially vulnerable to jokers teleporting in to say hi.

Regular nations can always use one more fort of course, but there's a point where they're already maxxed out on mage recruitment per turn, at which point additional forts become token defence which is not really needed as such : they won't die out even if they lose a 5+ gem province or two (or ten) for one turn, then take them back the turn after.
The same disruption really messes up blood nations however - either their hunters get killed, or they just flee but won't be able to hunt the turn they move back in, and even assuming the raiders are dealt with they'll have left a heap of unrest behind which further delays blood hunting.

Basically, in the late game a raid is a mere one turn's income disruption for a regular nation ; but at least 2, more likely 3 turns' for a blood nation. Which is why fortifications are even more appreciated than usual.