They do less damage than buckshot to standard tissue. The total tissue impacted is less than the bore diameter of the weapon. With buckshot the total diameter impacted is several times the bore diameter, and since the human or animal bodies are only so wide, excess penetration traded for tissue damage is wasted.
The result is buckshot destroys more cubic area of the size animals you would use shot against giving better performance.
Flechettes do have thier place though. They can pattern amazingly well at range, and they penetrate well, especialy against things that would deflect or defeat shot.
Most of the problems you see with flechette rounds that are sold is they are not used in a proper setup. Putting projectiles that are very narrow in diameter but relatively heavy in a shot cup designed for spherical shot means the shot cup is often perforated during firing allowing much of the energy to blow right through the flechettes and not be imparted to them. The result is they have a fraction of the energy and perform horribly.
You really need a robust shot cup or something solid placed behind them in the cup they will not penetrate on firing.
Another thing is most loads sold are improper charges. They are either undercharged significantly or they are charges mean for another load that don't get enough pressure with flechettes.
In short they are often loaded and sold by gimmick producers that don't know much about creating a quality product, just a novelty.
So flechettes are actualy better performers than many tests and various individual's personal experiences demonstrate. They are just put together by people that don't know what they are doing and the result is a round that performs at a fraction of its potential, leaving people with a perspective they are much worse than they are.
As far as legality, some of the more restrictive states have laws against them.
In fact I think even some places like Florida have laws against them, so thier legality can vary.
In CA they are outlawed, in IL I think they are. Most likely NJ and MA have them outlawed as well. There may be a few other places that have also adopted similar policies.
Another thing to keep in mind with them is that due to thier shape even projectile weights that are not traditionaly thought of as dangerous falling from the sky are dangerous. Thier shape will cause them to point downards on thier way back to earth and as a result thier penetration will be much higher than a similar weight sphere of shot, meaning even tiny ones are still a hazard falling from the sky.