I just realized a full hole all the way through the yo-yo means you have to be careful threading the axle since it could be too far to the “left” or “right” of the yo-yo and thus be not centered.
One Drop had this to say:
It’s actually easier and more consistent to make it with the hole. Tapping works much better when you can evacuate the chips on the other side. A “blind” tap is more difficult from machining terms.
But as mentioned, players don’t really like that “belly button” style.
I was thinking that steel on steel threads (with the metal nut) has to be a bit safer for long term longevity, but One Drop went and solved that so cleverly with their side effects system. But alu threads must be OK long term because there are still plenty of OD models – and virtually every other yoyo maker – use standard Alu tapped threads.
Also thanks for the reminder @Cannon I forgot Curvy Donut Man is one of the rare recent / new yoyos with the hole! I even have this one and I forgot about the hole!