Went from Keos to Zeros on my roadie. And went from Shimano MTB SPDs to Frogs on everything else. Solved my knee problems.
Huge fan of the frogs (for non-roadie bikes). The easiest clip in and out of any pedal I’ve used. Doesn’t have all the screws and overengineering of other Speedplay. Very simple design. Great for off-road and using with MTB shoes. Only problem I’ve found is that the cleats wear out faster than other pedals I’ve used. That and the price.
As for float, you feel a tiny bit but not all that much resistance in the Zeros. Way less than Keos. Your foot can basically shift around left and right on its own as necessary. But the Frogs are way less friction - it’s basically zero resistance in moving the foot around. So I find it even better on the knees than Zeros. But it takes some getting used to - feels a bit sketchy at the start. I also find I can’t use those always-on cleat covers thingos with the Zeros because it increases the float tension too much and makes my knees uncomfortable - maybe so keep that in mind.
Also, there isn’t any unclipping tension in the Frogs (you just move your foot enough so that the cleat literally slides off), so you can feel like you might lose the pedal when you first try them. In that respect the Zeros feel much more confident. You can move the foot freely but with no concern of accidentally unclipping.
Finally, Zeros are shit for anything offroad. I did a bit of soft dirt on my roadie recently and filled my cleat with mud and made it impossible to unclip. Literally impossible. I had cleaned the mud out, but it still seized up the moving parts of the cleat. I had to fall off my bike and then take my shoes off leaving them attached to the pedal. Was very annoying on this ride because I had to jump off to get across creeks and each time I had to fall off into a soft bush to get off the bike! Was like that until I got home and took the whole cleat apart to clean it.