Just so I understand, is your primary concern A) access, B) retention, or C) the negative effects of landing on it in a get-off?
The main reason I don't carry when I can when riding is retention concerns. I know it is unlikely, but I want to minimize my chances of the gun ending up on the road, so I want a holster with more retention characteristics than is typically needed in a "civilian" CCW situation.
However, I also feel I need to change my typical carry methods a bit (and not simply go with a holster with some retention features, but carried in my usual manner) because most of my typical methods would make the gun difficult to draw should it be needed.
I should add a third that I didn't even mention in my original post, concealment. When riding, the typical riding jacket is waist length, not longer (though it is cut a little longer in the back). I can see why many people give up on accessibility and go with shoulder harnesses when riding. So, a tuckable IWB (though, the shirt might ride up a bit while riding), some kind of pocket carry, or a high rise that will hide completely under a waist length jacket would be a good option for this concern.
When I first started riding, I worried about what might happen if I was thrown off and landed on my gun. I still won't do a small of the back carry option (though I don't like that one even when not riding), but otherwise, that doesn't concern me much anymore. If I go down while carrying, most likely I'll land on something other than my gun, and no matter what I do there is a good chance my gun will be thrown somewhere down the (or off the) road.
So if I was to rank my concerns for riding carry, my priorities would be:
2nd: tie between concealment and accessibility
Last: what happens if I go down while carrying