Good points. Now, realize that my Kendama experience is REALLY limited so I only have a small amount I can add here. But if you hold the Tama (ball) in your hand and try to balance the ken on the tama by placing the big cup on the tama then you’ll quickly see if there are weighting and balance issues. Some Kens have to be angled quite a bit to get it to balance, some can balance almost horizontally.
Also, I hear people talking about how sticky the paint on the tama is. I haven’t played with enough different kendamas to notice a difference so maybe others can help out there. Although I’ve found that the more I play the more dings my tama gets and that actually helps it stick a bit better.