yeah, not all o holes are hard to downstack.
The o holes that are difficult to downstack tend to show up when leaning too much on t spins. So in order to avoid it I think it is best to just directly downstack. The balance and the O holes was my belief that it's not good to lean towards offensive t spins too much. I downstack whenever there's an opportunity to survive and that brings the balance and avoids the difficult O holes from showing up. If it does then I would have enough time to skim through my pieces to downstack through it because my height is low. I used to play to kill my opponent a long time ago, but since that didnt work very well, I now play to survive, to take the win.
If you're aiming for combos, you should know that O blocks have a tendency to cut the combo off. so in a defensive situation when urgent downstack is needed, I check to see if there are any O holes or if it's a good field to combo through. and if there isn't I would whip out a quick t spin or go for just simple line clears maintaining my height in order to survive. It doesn't HAVE to be a tetris, t spin, or a combo to downstack. Going for simple line clears in urgent situations and having the motive to survive have saved me many times