The inseparable BOM structure is actually a very valid option and well designed, however the name 'Inseparable' is confusing and unfriendly in my opinion. Without digging through help files to find a detailed explanation, nobody would figure out what this BOM structure actually means or does.
I propose renaming the Inseparable BOM structure to something like 'Fabrication'.
If renaming is not an option (due to legacy usage), can I suggest providing & showing a detailed mouse-over descriptive tooltip for all 5 BOM structures. This would appear whenever the user is in a context whereby they're selecting a BOM structure.