If Boo Bash is any indication - Disney can't handle the demand, so it won't sell out in seconds/minutes since their phone lines get all tied up or their website crashes
With that said, the "early access" tickets might sell out but IIRC more tickets became available for people who were not AP or DVC - and then even after some the dates sold out, Disney added more tickets for most of the sold out dates. If you look now, some of the dates for Boo Bash are still available right now (end of august, early to mid-September) - but all dates in October are sold out. So I think it depends on what date you're planning on going on how hard or easy it will be to get.
I wonder if Disney will be smart enough to allow online sales for AP/DVC owners instead of forcing you to call and stay on hold forever.