It would help if you included some zone or location information.

Personally, I am in SoCal and look forward to the fall and cool damp weather and purchased several new fruit trees but that works for my zone.

Zone 4 central MN. It's already getting cold here. we're past the peak for seasonal colors.

I am in Zone 7 and I have purchased trees three times at the year end sales and all of them are still living. I have an elberta peach, burgandy plum, a fig, a crepe myrtle, and a blueberry bush. In my opinion, there is little risk.

I generally think its ok to get trees this late, just be vigilant about pruning any spiraling roots and definitely plant them in the ground ASAP. Consider staking them (just for the winter) as they wont have much if any chance of growing new roots to hold them in soil.