Fins are weird on a surfboard as they can help with upwind or hinder it. They are not all equal as toe in angles can be different, etc. On a skim the flatness is what helps the most, but you must have a big one. The bigger like with a surfboard the better it goes upwind and the more low end you will have. It's the same across all things including hydrofoils, twintips and kites where bigger equals more low end. Efficiency plays always a part, where more efficient is better. The generally rule for that is thinner and flatter is better. Hard to find kites like that though especially in the largest sizes.
You can edge a surfboard, you can also ride it with no fins or less fins. You can also ride a skim board flatter. You are right though you will edge a skim more, especially on waves. I would say the biggest difference is they require more balance, feeling very loose. If you watch that video you will see how Dom holds his hands out. This is how you maintain balance. Works good on a hydrofoil or surfboard too. With a kite you hold the bar with one hand. Good if it is setup so you can steer easy with one hand. With slack lines it is easy to push the bar where ever you want. Also good if you can ride with either hand on either side of the bar, and both hands upside down and reverse
So you can switch how and where ever you want.
I'll give you the skim board challenge, riding over 3x or 4x head whitewash. It's a sport in itself. Makes any wave riding on anything look easy. Like trying to stand on a bucking bull.