Yanmar entered into an agreement with John Deere back in 1972. Yanmar then built most all of the 40HP and under JD compact tractors until 1998. They still build the 790 and 990 and supply engines and other parts for the 4000 series. For a few years Yanmar sold tractors in the USA. They then agreed with John Deere to stop marketing tractors in the US and in return got another decade or so of contracts building John Deere's tractors. Take a look at a JD 770, 870, 1050, etc and it says built by Yanmar Diesel Engine Company, Osaka Japan.
The quality is outstanding, and unless you get one that is fairly worn, you should enjoy relatively trouble free use. Most of the people I sell Yanmars to aren't mechanics and do not need to be. Of course we fully service the tractors before they go out and find any little problems as well, so there isn't any work that needs to be done.
Most of the units in the US are Japanese models, not USA models. Some of the models were built almost identical for the Japanese and American markets, but most are not.
Any decent mechanic can work on a Yanmar. They are relatively simple. As stated above in a previous post though, some dealers will not work on them. It sometimes takes a few phone calls to get the right part and some tractors dealers don't want the hassle....plus they are a little miffed that there are so many of these sold, which cuts into their business
Where you buy from is important. Be very careful of "factory reconditioned" tractors. Many are rebuilt rough tractors. Buy from a reputable dealer that sells clean low hour original machines. Nothing wrong with a thouroughly serviced machine, but if it already needed to be rebuilt it has had a hard life.