I have read on numerous occasions that it can be difficult to get the "green card" to work in the US.

Apparently they are very keen to protect their nationals, therefore you need to be able to offer something which is not locally available on the employment front, before you can gain entry to work.

I think it is different if you work for a non-US based company with an office in the states, where in effect you are being "sponsored" by your employer.

Do not think the US is the same as Europe - it is very different.