has to give somewhere, and that something is likely to be your barrel
If you're talking about lead slugs - no, it isn't.
Try this. Cut a slug load apart. Push in on the side of the slug with your thumb....if it's a Foster (Forester?) style, the wall is beginning to cave a bit. If it's a rifled slug, the vanes are deforming.
Now try pushing on your steel shotgun barrel with your thumb, using the same pressure........
Not much happening, is there?
I've fired over 200 cast slugs through a 28" full hcoke winchester 1200, my first shotgun. It's fine. It has done no appreciable damage to the barrel or any other portion of the firearm.
Lead is softer than the steel, and the lead will give way first.
Now, sabot slugs are different...... I have been told that the hard plastic collar will NOT swage down properly and is considerably more likely to harm the barrel. Don't shoot those through a full choke.
As to why it isn't recommended - more recoil than open chokes.
Sometimes it harms accuracy.