Yeah, I'll have to add some retinue cap bonuses to demesne tribal holdings when this beta patch business is all wrapped-up. Still, though, those retinues are going to be expensive, and you aren't usually too rich when tribal. Having a lot of levies from vassals does not a professional army fund.
The +50% retinue cap bonus for Norse is a hold-over from TOG that Paradox hasn't thought to update for/since 2.2. I don't really think it makes sense as a bonus with the current retinue cost model (and vanilla and EMF now have the same cost model as of 2.2). Plus, there's really nothing to suggest a Norse ruler could/would employ a larger _professional_ retinue than, say, a Christian ruler.