Build quality is very good. Yanmar bolts and adjustments are checked twice before leaving the factory. This will be apparent from dabs of white and green paint on the bolts and linkage nuts.
I've personally ordered a rear-remote-hydraulics kit, Yanmar shipped it to my door.
Japanese Engineers have done customer visits here in the USA, TnAndy here is one of those customers.
Gaheadreck had problems with his SX3100 slipping out of 4WD on a steep slope, it was later determined the steepness of the slope exceeded the design specifications. Yanmar bought back his SX3100 so engineers could dissect the tractor and study the problem.
EyeCatcher has has some loader problems with his EX450, but I think some of his issues were induced by dealings with a bad dealer that installed non-factory hydraulic parts. He'll probably chime in if I misstated anything.
Probably worth noting, for every tractor dealer here in the US, Yanmar has several other dealers and service sites open to service boat and industrial diesel engines. That's one reason I went with Yanmar, there are about 6 dealers in my area that sell and service Yanmar marine engines. Yanmar has also opened a venture to rebuild their engines here in the USA, that should be a testament to supporting their products here.