RPGMemes

Yeah, best case scenario, It's mathematically the same as a +1 AC bump in that one more value of the d20 attack roll would be a miss than would be without it. Except the extra roll value is between 6 and 15 points above what would currently hit your AC, which is worse. If they already have to roll middling to very high in order to hit you, that 6-15 points could be higher than it's possible to roll without criting (i.e. >19)

Even assuming your AC is abysmal, like 10, if you roll a 10 on the die, you now get misses on a roll plus modifiers of 1-9 and 25 (excluding crits). If the attacker has total modifiers of 5 or less to hit, that 25 isn't doing anything at all as they either couldn't hit it or would crit to hit it. So in order to keep your extra point relevant, you would want to save a low roll, ideally a 1, for an extra point 6 points above your AC. But even that gets less useful the higher your AC is. Too high on either your AC or your roll and you have the same problem, the ring does nothing.

This ring is maximally effective the lower your AC is. And even then, best case scenario, it has the total effectiveness of a +1 to AC.A ring of protection already provides you that mathematical advantage. Plus ring of protection is less rare.