The frame mount is a very tight fit on the Twin Cooled bikes. The M8's do not vibrate nearly as much as the Twin Cam's, so it is really not necessary to use the frame mount on them.
On my 2014 Twin Cooled Twin Cam it would have been better using the frame mount, even though it is a PITA to install, simply because of the vibration.
I've not had a fan motor fail due to vibration, but I have had the rubber isolater fail at about 40K miles. It was a 30 minute job to replace it. That was on a Twin Cam.
With the lesser vibration on an M8 I do not anticipate any failures from the horn mount position.
Kevin