Excellent question! As it turns out, I did some experiments on that last year, or maybe it was two years ago. I was primarily checking how far down the rear fender needs to extend (all the way). Just some tests, not rigorous science.

I started with several clean white t-shirts, my road bike and a cruiser we had with full fenders, and muddy puddles after a rain. My conclusion was that you need full length fenders (curved all the way to vertical) to prevent splatter on your back, but there was no discernible difference between the curved commercial fenders and flat ones like these. I didn't check the front - I'd expect at least some difference since we have more exposure as it moves left and right, but I do know that it's effective. I have sturdier aluminum ones on my main ride.