Another Dutchman here.
Leader: Well they couldn't have chosen anyone else without looking like fools, really. Not picking William would've been like having John McCain as the leader of the USA. I mean we've had a few other great folks, but most of them actually weren't leaders.
UU: It had to be something naval, and the Sea Beggar is a great choice. As it's the only Privateer UU it also stands out, most other options would've been a Frigate replacement where it would've competed with the Ship of the Line. It's iconic, it's powerful, it's great.
UI: I think this one could've been implemented slightly better. The current restrictions do feel kind of odd, especially the whole Flood Plains deal. It's not a bad choice to implement, nor is it a poor thing gameplay-wise, it just doesn't seem to be implemented the best way it could've been. Still solid.
UA: It's fitting, that's for sure. High standards of living + Lots of trading well represented.
I would've however liked the Dutch Culture/Maritime Tradition slightly better represented, given our history of Great Artists and Admirals, but as it is, I'm more then happy with the Dutch.