Part of the issue is tackling though... hard enough to go from League to Union with the difference in tackling, hold+release etc. Never a guarantee a great League player will be a great Union player, and same the other way around.
If an NFL skill player was to play they would likely be C, W, FB... all three you have to be able to tackle so well or you aint making top level no matter how athletic or skilled you are. Ofc it can be taught... but a lot of guys in UK have made Superleague debuts, got a few tries over first ten games, made offensive impacts and then been released... as their position defence and tackling was mediocre. And then at C, W, FB you also have the kicking issue. Hard to be really high level without being able to at least place a ball well. And then IQ to see best passes... when to kick... where gaps will develop... when to try and hold it up and get support, when to take it to next phase on purpose.
Of course I am 100% sure some guys would be able to adapt and be good players. But some would just suck or be mediocre. I guess that some random special teamers here and there in the NFL would be international level after a year, whereas some NFL stars would not be able to play in even Superleague or NRL.
I am 100% you know all that and thought of it though!
Played with/against a few future Superleague players... best player I ever shared a field (in training, he was year above me so never got to play alongside him sadly!) with was probably the smallest and least impressive physically at only 5'6 and ~160lbs at the time (now 190ish!)... just insane game IQ and kicking game along with great athleticism. (Kyle Eastmond... but I mean, might have been best player in UK in league for a year... switched to Union and is just "pretty good".)