Curve Optimizer is essentially per core undervolting.
It’s possible that something is different with Zen4, but a behavior that I have observed on my Zen2 CPU (and IIRC was confirmed as intended behavior by AMD’s head of technical marketing) is that when an offset undervolt is applied the boost algorithm starts requesting up to 0.05v additional voltage so that it ends up with the same voltage after the undervolt. If that doesn’t work then it starts clock stretching (which reduces performance and reduces the voltage the CPU needs to maintain stability).
I think the reason for this is so that if the motherboard has poor voltage regulation or something like that (anything that effectively undervolts the CPU) the boost algorithm will make sure the CPU still gets enough voltage to remain stable.
Curve Optimizer however instructs the boost algorithm to accept the reduced voltage and accept possible instability.
On my Zen2 CPU when I apply a 0.04v undervolt the reported voltage and power consumption stays the same, as does performance.
However when I apply a larger undervolt (ex. 0.06v+) it starts clock stretching. I see similar (sometimes slightly better) performance in Cinebench and average framerate in games, but the 1% lows are severely impacted and stutter increases dramatically.
So undervolting without curve optimizer is definitely problematic on Zen2 and I think it’s also problematic on newer Zen CPUs (definitely seen reports of it on Zen3, not 100% about Zen4).