The river bonus might be their best power, I'm not quite sure yet. I've gotten some use out of it, but not as much as I would have hoped.
The reason the river bonus is easily their best right now is that beyond giving you more production from IZs, (+4 with the card is always welcome!) they allow you to almost guarantee campus and theater squares get +3 with good placement, as long as you have a river; this in turn means it's possible to get Rationalism or Grand Opera's adjacency requirement fulfilled nearly everywhere. That's +50% from buildings that other civs are forced to use world wonders or mountains (i.e. get lucky.) It also means you can have a much more consistent era score bonuses for the 3 affected districts, if you need it.
Well, if a Polder count as a flat land, the Netherland could transform an Archipelago map into a dry earth.
If the requirement was [2 flat land tiles
and (a third land tile
or a polder)] then you could never fill in the whole sea. You'd effectively be able to line much of your coast with them, but you could never place a polder that wasn't adjacent to two actual land tiles. It's an easy tweak to the logic, and wouldn't require sacrificing the sea tile aspect. Again, since you also have to drop harbors and water parks and other things on those same coast tiles, it's not as crazy as it sounds. Honestly, I think they should let people do it- we allow some crazy stuff already. (Cough outback stations.) We can always nerf the civil service and replaceable parts boosts a little, perhaps +4 gold down to +2 gold, and +2/+1 adjacency down to +1/+1 would be a fair exchange for getting to build adjacent polders in the first place.
but if you think the polder have strict placing requirements, try placing a chemamul and come back here.
Doesn't the Eiffel tower work with Chemamulls? I agree 4 appeal isn't so common, but between coast, wonders, mountains, NWs, there's more than a handful of spots
per continent you can build them on before Conservation (unlike polders right now.) Admittedly you have to be careful of mine placement, but remember that Firaxis also nerfed Australia's district bonus on breathtaking appeal too; presumably because it's more common than one might think.
I hope to play a map like fractal or archipelago to get my polder game on, but most games are continents or pangaea. A lot of civs become better if you optimize the map settings; abundant resources for Hansas, wet (more woods/marsh/jungle) for Mbanzas and stave churches, etc. And of course Indonesia on islands maps is just cheating. I think we should consider the standard settings for balancing. Right now, the dutch may be a perfectly fine civ, but you just don't get to
use one their most exciting aspects, and that feels really un-fun to experience in game.