Did we ever figure out if premium cubes are better from rare-->epic?

People aren't going to charge in the first place if it's limited.

Experience shows otherwise.
Two years ago there were limited quantities, people raged and said nobody will charge the next year.
The year after, people got all excited and charged tons because "Nexon said Cubes and AP resets would be on sale!" When hindsighters tried to say "but we knew this would happen", they said "but there were these prizes we needed to unlock, and that's an implied promise there would be enough for everyone that contributed to the unlocking" and all sorts of baseless optimism.
I bet their profits this year will be very nice too, even with limited quantities. Nexon operates on the "there's a new one born every day" premise, and it works for them. I'm sure they will have a way this year too, to coax people to charge ahead of time, end up with nothing, and buy everything at full price later.