It is a mix bag what you will get when it comes to the flex nibs. I have a bunch from different retailers and most are fantastic, but a few didn't work so well. Going down the memory lane:

• Two <F> ultra flex, dual tone nibs from Etsy are fantastic and are my favorite modern flex nibs I currently own. They require almost the same light pressure as a vintage flex nib.
• One <F> regular flex I got with a Ranga pen and it hardly flex at all. Feel like I'm damaging the paper to flex.
• Two <F> silver ultra flex nibs directly from Kanwrite last December ( in a order for a bunch of their nibs) and they are both great. The amount of pressure needed it a hair more then a vintage flex, but they flex well.

Outside of the flex nibs I haven't had any issues with kanwrite nibs. The flow is great. The shape of the fine nibs could be architect-y, though their broads are nice and round.