Attack Damage Converted to Health applies differently depending on what it’s source is. Spells with it innately will heal that %age of the damage they deal. It is tied only to that specific spell, and applies to nothing else at all. The actual stat, added directly to your character sheet, will only heal that %age of the damage your WEAPON does. This means if you do 100 damage with your weapon with 10% ADCtH, you will heal 10 hp. If you use Savagery, with 150% weapon damage, and it having a base of 300, with a 100 damage weapon, you will heal for 10% of the 150 weapon damage, or 15 hp, and the base 300 damage is completely ignored.
As such, ATCtH is worthless if you are not doing any damage with your weapons, say a Panetti’s Replicating missle build, sorta weak if using an auto attack modifier focused mostly on flat added damage, like say a Firestrike pistol build with pure focus on Brimstone and Static Strike. Meanwhile, a 2h Savagery build, with 150% weapon damage with 300+ base damage weapons, heals significantly more per hit. Now if you are attacking with your weapons, it is never worthless, but it’s value scales directly with how much weapon damage you do. It also doesn’t work on bosses unless you reduce their resistance to lifesteal somehow, be it a targetted resist shred like Haunt relic, or a general one like Manticore devotion proc.