Some of the kits may be snake oil, and all results may be short-lived, but some kits are better tan others, and there are things to know. I'm certainly not a headlight expert, but I've done a few and can say that results vary, and the process matters.
I may be wrong, but I believe most OEM headlights have a protective coating over the plastic when they are new. This coating can deteriorate. On top of that, damage from sun, water, chemicals, road hazards, sand, dirt, and more can all cause small scratches which then lead to further oxidation.
These kits in theory, and some in practice are meant to remove the oxidized plastic and allow you to polish the plastic underneath back to a clear appearance. These can work at least in the short-term, if the damage is purely on the outside of the lens. Why I say short-term is that unless you apply a good coating with a UV protectant, it seems that freshly polished lenses tend to oxidize faster.
So, if you can physically remove the oxidation with sandpaper or some type of abrasive. Polish the lenses. Then apply a sealant/protectant, and if your vehicle doesn't get much sunshine after, you may be good for some time.
I've recently restored mine, and they look much better. However, I'll be replacing the lights up front soon anyway.
He's right, they should be clear. As you've likely found out, pretty much all the headlight restoration kits are snake oil at best. The one I tried from Cera-Kote actually made my headlights worse. On my Roush, I was lucky enough to buy genuine Ford headlights from LMR, however these are discontinued. After searching ebay, I finally gave up and ordered aftermarket ones from LMR for $142 and they are nicer than I thought. Of course, right after I bought then, I found some US-made ones I would have rather bought on ebay, made from Original Ford Tooling for like $240.