"Colonsay does not have a good sheltered harbour."
Quite so but Coll does, and so does Tiree (to a lesser degree). You can't predict when the ferry will fail to line up adequately to the linkspan, nor indeed whether the Islander will get in. As a flyer you will know that it depends not just on wind direction ( not a problem on Tiree to the extent that it might be on Coll), but also on cloud base (not a problem at all for Calmac).