Which was 3 years ago, but Ukraine is surrounded by Nato allies. In the event that the US steps in they would use one of the numerous Airbase, if not create their own ones, in their allies countries anyway. Making it a moot point. I know a C recently crashed near China and fell into the sea but that seems like an isolated incident. I remember hearing about the helmet issue a while back, but I think that was fixed.
You've got to remember that even the f16 had issues that needed to be worked out, but they did that in the 70s. The f35 is a revolutionary aircraft and atm has no peers. It's why more and more countries are signing up to order them even now.