Frame mounted nerf bars have long levers (mounting brackets) between the frame and the rail/tube/slider which allows it to flex/bend and apply a great deal of leverage against the frame mount. Which is one reason the OP's mounting bolts are coming out from where they are screwed into the frame.
Tub mounted sliders/rockers have zero leverage against what they are mounted to and they are held to the tub with something like a dozen bolts that can also be reinforced with an interior stiffening plate to make it even stronger.
Not only that but the mounting bracket that connects between the frame and tube or slider just gives two more points for trail obstacles to grab and hang up your Jeep.
You won't find anything but tub mounted rocker guards on the hard core trail rigs, especially rock crawlers, because the frame mounted slider brackets hang up on obstacles and they aren't nearly as strong against rocks and other trail obstacles.