Before nerfing Norns take a look at actual mythical costs.
Gathering that 600 food is non-trivial. And requires significant usage of space for farms you need only for this.
If you use trade - it will always trade above 300 apparently so you won't get to 600. I already mantised this.
There is a by design military disparity between Greek and Norse. Norse get better mythological units - I don't see that currently. Norns are one of ways to actually build some of these without devoting significant part of your economy to that.
Well, the main issue, to me, is: It's *only* benefit without real risk (and with not much cost). Even the weaker items generally have some risk, but this one is A: without risk, and B: super easy to use. It's almost a small sort of "win" or "fix" button. Need quick extra strength to rebalance by enhancing the Norse side to match the other guys? Use Norns, and then carefully decide how many and where you need these guys.... no risk, no trouble.
And the difficulty of getting those guys really does depend on how you set up. It's pretty easy to set up a city that's JUST fishing spots, for instance (and the bare minimums for keeping crime there down). That adds up pretty fast.
I'm not saying it should be super freaky dangerous to use, but there should be SOMETHING. I'm just not sure what it is.