Probably won't affect many folk. There is an anomaly on Garmin plotters when going through the Cuan Sound between Seil Island and Luing, just at the point where you have to keep well off the Seil shore (to avoid the rocks)and stay in fairly close to the wee island with the marker on it - the plotter shows the boats position to be to far to the SE. If you take the correct line the plotter shows you sailing over the island, i would therefore imagine that if you followed the plotter it would in fact take you over or very close to the submerged rocks. Maybe not to much of an issue in a shallow draft rib but i wouldn't want to test it out.
Anyhoo, if you are passing through and have a Garmin plotter bear this in mind, I have to also say that this is the only place that i have found it to be off.