If you knew exactly how much (weight wise) to add after the tire is mounted maybe they would work. Suppose you don't have enough added to begin with to obtain a good dynamic balance? What about if too much were added?
Then you have internal tire mold nubs and low spots for them to gather in. What then? What effect will the little balls of rubber that wear off the inside of the tire cause with their movement inside the tire? Condensation cause them to clump together? I guess you could fill them with nitrogen.
Fwiw, I've got some of that tire liquid balance and puncture stuff in my car tires. The first trip out and the thing rode like it was on square tires for 40-50 miles at speeds no more than 30 mph. It was awful and I was tempted to run to my tire guy and claim insanity and have him get it out.
However, it soon got better in around 80 miles or so (they said it would, but I didn't believe them as it really hammered going down the road), but there still are times the thing gets out of whack for whatever reason and hammers a bit on the freeway. If I pull off, stop, and slowly roll at slower speeds and then jump onto the freeway it will settle out and the steering wheel will smooth out. Only nice thing about the stuff is that air pressure stays up for months now. I may have nails and screws in the tires, but it's still rolling along.
I bought some at the same time for the motorcycle (they say the chemical's compound is different between a bike and a car), just never added it 'cause sometimes the car goes out of balance. I left the old lead weights on the bike and just painted them to match the rim.