A free float can't make a mediocre barrel shoot well. Every barrel has it's best group, what the free float does is prevent things from making it shoot worse. It keeps sling or rest pressure from moving the barrel around and distorting the sight plane from the actual point of impact. Mount the front sight to the tube and it goes right back being distorted, altho less. Free floats can be more rigid than a barrel but there is no guarantee, and nobody tests to see how much deviance their is.
Because of that flexibility no scope maker warrants their optics if you bridge the mount from upper to front rail. They claim that the tube moving around will bend the scope tube. If you are building a rifle to a high precision, they help. Using them as just an accessory mount, not so much. The cost is 10x higher compared to screwing something down onto a handguard.
On a hunting carbine, there isn't much to gain, 2MOA will hit a ten inch circle at 500m. Good enough for deer. A woodchuck, prairie dog or antelope gun would benefit but the need is less important than a good barrel in the first place, with tuned ammo and a great scope. Shooter skill takes precedence to a lot of addons. Shooters were hitting prairie dogs in the old days without free floats and iron sights.
I put one on my hunting carbine and it was lighter, but the aluminum required a glove even in mild weather to keep from sucking heat out of my hand. When still hunting or stalking for hours, it can make a difference compared to a poly resin handguard.
Because of that the AR pistol is getting B5 handguards. It's not a 500m gun and it certainly will be used in cold conditions. No point to spending at least $75 more to have less.
As for the M4 float, that was an institutional decision by committee to provide a universal mount for all the various accessories in the military. The barrels are milspec 2MOA, no accuracy advantage was intended or needed. Many now recognize that they are an expensive and relatively heavy answer to mounting an accessory that needs about three inches of rail, if not directly in an existing hole on the tube.