The way I see it is this:
Even if the skillup rate was as high as 10% without boost (I have no damn idea what it's supposed to be and
I'm being generously high with estimated success rates), it's still terrible. Even if the skillup rate is, logically, 25% on a 2.5x boost, it's still a 1/4 chance, and with the time and effort it takes to get skillup mats in the first place, as well as the fact that some mats
need to be evolved first beforehand, it's still shit.
So really I feel it's completely random and the multiplier is an illusion, really.
Let me remind you that I went 0/20 on
one Ronia skillup on x2 rates and today I went 2/2 on Orochi skillups.

This is why I've grown enamored with hunting +eggs instead recently, because there is a definite, tangible, visible boost when I fuse them, and it's a 100% chance of benefit when I do so. It just feels more rewarding than endlessly pursuing skillups only to be disappointed. It's also the reason why I've taken Parvati from 0 to +130 in the span of about two weeks. :V
As far as skillups go, you may as well just farm for pys instead, you'll probably end up with relatively similar rates anyway. At least they're universal for their respective attributes and able to be collected at any time, not during any short event. :colbert: