Originally Posted by
59Impala
Nothing to do with „Swiss“ or „bad experience“, you should ALWAYS book seats with the operating carrier to have stuff under control.
The OP did that. Only the return flight is on UA, as I read the description.
I will say that my personal (limited) experience with UA is that they feel seat assignments are just a suggestion. I've had seats changed and dropped with them at a far higher rate than any other airline. YMMV.