I was thinking about getting a Toyota before considering Jeep, and I read that the undercarriage of some Toyota trucks can get pretty rusty without protective coatings. Does Jeep have any significant rust issues on the underside?
I wouldn't say the JL has a rust issue. That said, you can give yourself rust issues in certain environments if you're careless. I'm out by the Bonneville Salt Flats. If you're out in such a salty environment and don't bother to clean it off, you will have rust. But that's true for all vehicles.
I picked up a 2020 Rubicon a few weeks ago and was surprised by the amount of underside rust I could easily see. Now granted, none of this seemed serious (just now) and was mostly concentrated around the edges of where sheet metal was welded, on the frame or chassis but was still surprised nonetheless.
Updated to include images which don't actually look as bad as it is when viewed by the naked eye.