The reason Sabot are considered useless is because they're a limited ammo weapon that takes out regenerating shields. It's really easy to take down shields with them, but you only have 6 shots so besides for a free overload or two it doesn't help. Harpoons are better as once the shields ARE down they do massive amounts of non-recoverable damage.
well, really depends on the weapons you are using and on the enemy ship...
some ships are actually incapable of bringing down enemy shields because they don't have the flux capacity for it,
some enemies just shrug it off (happens even more with some mods where frigates with 0.4 flux/dmg exist)
but can already happen now and then
and especially bigger ships have trouble in bringing down shields because smaller ships can just fly out of range when they need to
also there are some other factors, harpoons are weaker against PD (sabots are too fast to shoot down)
and the damage of harpoons is just semi good...
most ships don't even have that much armor and sabots deal the same amount of damage against hull