Rod,
Yes, if the backside is not accessible then they are not appropriate. Riv-nuts are pretty good but Nutserts are not very good in fiberglass. The two types are frequently confused and referred to incorrectly. Riv-Nuts are similar to Pop rivets, they are secured by flaring out their shank behind the panel. They were developed early in '30s by BF Goodrich for attaching de-icing boots to the leading edges of airplane wings. A Nutsert expands radially into the by wedging its read half into its front half. These are not good for thin or soft materials. They are exasperating when they fall apart and the rear half falls into an inaccessible place.