Average Islands Scale tends to introduce overlap between islands. This causes issues when trying to pack mirrored meshes, especially complex ones with hundreds of islands.
For example, let's say we unwrap Suzanne like this. The face, ears and eyes are all mirrored, and intentionally overlapping.

If we run Average Islands Scale on this, it causes all of the islands to overlap. This interferes with add-ons such as UVPackmaster, which optionally treat overlapping UVs as a single island.

My simple fix is to add an optional scale method called Shrink. It scales the UVs relative to the largest island, instead of the average area. This solves the problem in all but a few edge cases.