Since there were surprisingly no details on what these natural wonders do in the patch notes, I spent some time looking for them in-game to find out.

• Lysefjord - Naval combat units that enter adjacent plots are granted their next promotion.

• Eyjafjallajökull - Adjacent land plots yield 1 Culture and 2 Food.

• Giant’s Causeway - Land combat units that enter adjacent plots receive the ability 'Spear of Fionn' ( 5 combat strength).

Thoughts:

Lysefjord seems pretty bad if only because of how weak naval combat is right now. Could be really good though if naval combat becomes relevant in the future.

Eyja is basically if Piopiotahi and Mt. Kilimanjaro had a baby with no money. Super strong, probably the best additional yield natural wonder in the game right now outside maybe Torres del Paine. You get 6 yields from the 2 middle tiles, and 3 yields from the other 6. This pretty much makes a tundra city super viable on its own.

Giant's Causeway seems like the best of the three at this state of the game, at least IMO. 5 combat strength on all your land units for free with no attacking restrictions to boot (like in most class promotions) is pretty damn good. 40 combat strength War Carts seems like some kind of nightmare I don't even want to think about.