Finning wrong is a matter of opinion. I have a set of Twin Jets and do use them on occasion but find them not efficient for the primary kick that I do (the frog kick). There are many divers out there that have absolutely no use for seeing a flutter kick taught. These divers will not use splits.That is true to a certain extent. Split fins use a different type of fin kick than your regular fins. With split fins you need to do more of a flutter kick. The Votrices created by the short finning motion is what moves you through the water. That's why they're better for people with knee problems as their's less stress on the knees. Most of the time people say they don't like them is because they're finning wrong.Try swimming with your fingers spread out, then try it again with them together cupping the water. Which gives you greater thrust?
My dive buddy uses splits and uses them with the designed flutter kick. I must admit that I sometimes have trouble keeping up with her and she is barely kicking. So, there is something to it. I also know from experience that my air consumption in my splits tends to be less than my DR paddle fins. But the paddle fins are still my fin of choice for the reason that I get more control of my position in the water (ie. turning on a dime and stopping) which is highly important to a photographer.
Still, I don't criticize split fins as see more sides to the story now. Split fins have advantages as do paddle fins. You just need to kick a little different from one to the other.