yes

canonically yes in the game however no.

In a sense, yes and no.
Sans took me FAR more attempts to beat than Undyne the Undying. Sans took over 100 while Undyne took less than 30.
BUT
With sans, I can consistently beat him within 3 tries, even without any practice after months of not playing. I've memorized everything. It is no longer a challenge.
However, with Undyne, every time I fight her it takes me 10+ tries.
I don't really know why, but that seems to make Undyne harder, since I always take many tries, but as soon as you beat sans, it's easy.
Does that make sense?

yeah, i see where you're coming from

the muscle memory adapts after a while

Hmm

You aren't on scratch by any chance?

im not, i use a (100% legit not pirated) real version of undertale

Lol

I asked cuz a scratch freind of mine said this same thing

yes agreed even though i haven't fought sans yet
i hate the green attacks and that one attack with 6 spears in opposite directions

IK

istg green soul gon be the death of me

we talking genocide?

i have no idea but probly

where do you fight sans in pacifist or neutral

Depends on the person
I can beat the Undying without healing consistently, probably because I'm good at green mode. Honestly, Asgore puts up more of a fight to me now. Snas is still kinda hard for meh- but if I heal all the way I'm good

this sentence looks similar.....

It depends on the player. Me, I disagree with that notion. I have never died to undyne. All you have to do is have quick reflexes. For practice, try beat saber or a dance of fire and ice. SANS on the other hand is suuuuuper annoying and it takes a while to beat him. The time it took me to beat sans, I could have beaten jsab.