You see steaming pits at every ski resort....when people take their Jackets off

That is just wacky.

in fact, it is more than wacky, it's scarey

Quite literally to leave before they arrive. Niseko doesn't start to crowd up until the second or third week of December. Little known fact that the resort generally opens in the last week of November.

Pros of an early visit are no crowds and cheaper prices.

Cons are the risk of a catastophically late snow start, as there are very few snow making machines at Niseko. Also most bars, restaurants, and services won't be open. This can be a plus as most of the people you will hang around with are resort staff instead of tourists.

This may also provide some interesting reading on season starts throughout Japan:
http://www.snowjapan.com/e/general/winter-sports-in-japan-the-season.html

Also, avoid the New Year holiday season - You have to wait for - oh, I dunno, a couple of minutes for a lift! Right pain, that is!

Mind you, compared with Oz resorts, the longest wait I ever had in Japan is about one tenth of the wait at Falls Creek express.