The frame was redesigned because the rubber-mounted motor imposed shake, not the bat wing fairing.
The frame contributed to the overall problem of shake at speed. Not the sole source of problems. Head shake not
always causing slappers. However frame shake could cause slappers at higher speed especially in long sweepers.
The bigger issue was fairing related. Made worse by frame/suspension induced shake, sloppy head bearings, bearings not
torqued properly, and fairings not adjusted properly.
There are too many hundred reports of this. Just do a little searching on a HD specific forum.
I dealt with this the entire time I had the bike. Definately was a issue for some.