I think the advisor is op compared to what the War Chief Comontorios does. Maybe its not op but compared to the pop advantage seems not to be fair. But you can discuss about Norse does not have a Woad Raider. But Norse have the better Zerk compared to Champ, etc.
So, Critical thing here - You should not EVER compare Horselord Finnulfr to War Chief Comontorious - Or to ANY advisor that Norse can't use. Sure, it's easy to draw comparisions between the two of them, but Horselord Finnulfr does
NOT compete against War Chief Comontorious - He competes against Neutral and Norse A4 advisors, such as Boudicca, Theode, Beorix, ect.
The only reason to nerf Finnulfr would be if, as Norse, Horseman builds suddenly dominated the meta - No other unit could come close in comparision. This is the main reason why Brann was nerfed for Celts - He made Woads
too tanky for their extreme damage output, meaning that any other Celt build was essentially inferior to just spaming Woads. The amount that Finnulfr buffs Nordic Horseman has absolutely NO effect, nor any standing, nor any relation to how much War Chief Comontorious buffs Horseman and Carpetoms.
While Norse does have a bit of a PvE unit balancing issue, with Berserker's likely being too strong in comparision to other Norse units, Norse as a civ isn't dominating the PvE meta.
Finally, War Chief Comontorious is actually quite strong - Keep in mind that Woads have the top damage for a PvE unit in the entire game due to their AoE charge and high baseline damage - It's so high, in fact, that the second highest damage output unit isn't anywhere near close to the burst Woads can do. However, Woads are a glass cannon - If you are a player who prefers a tankier playstyle, or a frontline/backline playstyle, then you use War Chief Comontorious - Carpetoms deal a signifcant amount of damage for their pop, especially when you consider they also have splash, and the buffed Horseman make for a fairly tanky frontline combined with the Celtic Horseman's increased pierce armor. Keep in mind that Comontorious tends to sell for quite a bit more then Brann (And much more then Brennos), the other two Celtic advisors it competes with.