Every mustang has been a unibody along with every F-body and about every car made since the mid 70's (except the 80's monte/buick regal based cars and a few others).
frame off still applied there are just 2 sections of frame to remove. I remember in the early 90's a buddies dad saying "who the hell is going to want and restore anything from today" while we were stripping his 64 malibu SS for a frame off.
Granted 90-95% of the cars then and even to now are not going to be around in 20yrs. However, to answer the question my buddies dad asked well any mustang, any f-body, regal/t-type, vett., some of the nissan 240-300's, supras, vipers, conquests, talon/eclipse as a list of cars up to the early/mid 90's.
I would not say they are not as strong as a full frame car but just different...both have +/-. 99% of the time a car is totaled it is due to the air bag going off due to cost of replacement of those systems at say around 2k+ that is a big chunk for such a small part of the repair before you even address the accident damage.