I think the biggest difference in a set is the wood species. Old Time Games uses a kiln-dried poplar that (for a recreational player) will last a very long time.
The Madwood sets appear to be Southern Yellow Pine, which is a bit softer, and takes damage over time. I have also seen kubbs made from pine split from impact; I have yet seen a poplar kubb split. Remember, a kubb set is a consumable.
Sets made of cedar or hem fir are even softer and lighter, and end up splitting and cracking under even light usage.
Secondly, with me being a driller, I look at the weight, size, and the routing of the kubbs as very important. Light kubbs bounce more, larger sized kubbs are easier to control, and the routing edge can make a difference in the angle at which you drill.
I am also big on keeping work local in the United States, so for me, I would rather buy American. Some (most) retail sets are not made in the US, which is important to me.
I am glad it's usakubb.org on the back of the boxes. If it wasn't usakubb.org, it would of been some other website...
But I do not see a flaw necessarily with any kubb sets. Regardless of the wood used, the company, or the routing technique, kubb sets being played on should be looked at as "viral" marketing. A game played on ANY set will become a game loved.