Re converts, league players are more specialised at running the ball, breaking the line and tackling.
The difference between any edge 4 defenders which is 8 of the 13 on the field, is not that huge (except for the ball players of course), either way if you are very good at that in league chances are you will make a decent fist of those types of positions in union.
Similarly union players who specialise in those skills and are very good at them, will make it in league - See Semi Radradra.
Middle third players in league see burgess / thorn will have a hard time crossing over to union, but should have very high base fitness / power athletic levels that helps in portions of the game of union, if they stick around for long enough , (or have a background in the game like thorn) they should be able to give it a decent go.
It would be very difficult for a 135 kg prop who doesnt have the requisite burst speed but has very good scrumagging and possession type skills to ever be any use in a game of league. Obviously if they have the speed they could probably transfer.
Also, union sometimes has alrounder type wingers who can kick a bit and do a bit more. In league your winger these days has to be powerful for the first hit up, be able to catch very good, but really passing and kicking they are not going to use much on the edge.
I think fullbacks though given their role as usually the most talented 3/4 in the league side, covering by far the most ground, having the most ball skills generally and best 1 on 1 defence should be a very good convert to sevens rugby.