They are every bit as good as the so called big three.
Iseki makes the compact tractors in Japan (to AGCO specifications) and have been for a very long time. They are a huge company.
Agco is the top selling tractor maker in the world and in the top three in total equipment sales along with Case New Holland and John Deere. Kubota has no where near the amount of volume of equipment that AGCO sells.
Go check them out (wether you look at the Massey, AGCO or Challenger models). They are just as refined and ergonomicaly put together as any tractor (big three included). In fact I think many of their models are tops in their class.
I really do not understand the BIG 3 concept. Leaving a company out like AGCO is pointless. It would be like saying Ford or Chevy or Toyota could not build a compact 2 seat car, even though they are huge producers of cars.