I think the +1 level traits (sov, faction, or fortress improvements) as they are currently implemented not really worth taking.

Basically they just give you 10 or some more XP to reach the second or some other low levels.

It would be much better if they would have worked like in MOM, where your unit has the same amout of XP, but with that XP they are several levels higher, than a normal unit.

So for example soverign without Veteran would start at level 1 with 0 XP and reach level 2 at 10 XP, but another soverign with Veteran would start at level 2 with 0 XP and reach level 3 at 10 XP.