If the nut that holds the tube in place is seized, what makes you think the crossmember bolts wouldn't be seized as well? Snap those off and you're screwed.
The threaded inserts can get damaged. I zip them off with an impact and then use a thread chaser. Sometimes the bolts strip on removal. Old cars and all... Rusty cars are the worst. I won't work on them. Not worth the headache.