Having a cow in a fight is always an advantage, unless the province is crap (i.e. no wpa). People have mocked our faery cow for some inexplicable reason, but she has a comparable income to the entirety of simians merchant kingdom and this is the only reason tacticians are able to compete with the superior merchant personality IMO. The only "disadvantage" you get is that most kingdoms try to protect their cow and play for its long-term growth, and this limits your actions some. But this is a choice, not a true disadvantage.
However having 5M of networth on a cow province is not as advantagous as having 5M of networth distributed across your core. If you're warring a kingdom 7M nw bigger than you with a 5M cow it just means you were in decent topfeed range and out of dragon range. But if they had an even core they *wouldn't* have been 7M bigger; the true advantage of a cow (especially this age with double explore costs) is that it lets you use your pool cheaply. One kingdom having a bad age and another having a good one is a much bigger boost than a cow will give, and with hoh/freakstyle that was probably the case.