The idea on that is that the stress point is different, and maybe a point not actually able to handle the load.
The difference, of course, is that on a Gen-I the engine mount bolt (best place to put a slider) is not accessible through the fairing slot, you have to put a hole in the fairing. The "bolt-on" sets have a bracket that mounts onto the frame and they hold a slider that
does fit through the fairing slot. That puts the crah load in a different place, maybe a weaker place.
There is one bike a couple of years ago that used the engine-bolt sliders that actually broke the bottom of the bolt hole off of the bike frame in a crash. I do not recall if he was able to get the frame repaired with a weld or if he had to replace the frame. Unique situation, though, only one that I know of.
I have the R&G sliders on my '03, have actually had to use them twice. First one, the bike slid off the road and the slider dug in to the dirt, bent the bolt when it hit a rock ro something. Second one was actually faster, but didn't go off the pavement, just ground the slider down. First one didn't save the fairing, second one did. No frame damagae in either case.
As for how they look on the bike, I have no problem with them. Lots of folks don't want to put a hole in the fairing, but you can't see the hole, so it's a "so what?" to me.