The only problem I see with military tribunals held in Ukraine is that we would have to watch on TV and not be able to attend in person.
I don’t know where military courts would be held; Putin has said there would be trials for those who committed war crimes in Ukraine. From my understanding, he was referring to Ukrainian nationals and foreign mercenaries. Don’t know or remember what he said, if anything, about trying US or other nationality people for crimes against humanity.
Regarding xone’s comment, the UN would be the last entity on earth to hold trials, many of them need to be in the dock.