True, but at the proper impedance a 100w cab should hold up against a 100w head, granted you dont over push it with an overdrive then max it's output. People used them for decades like that without issue. But being merely 25w per speaker, it doesn't take a whole lot to blow one, especially being driven by a 100w amp.
Due to that issue alone, while I like the sound of Greenbacks, I'd rather not use them personally. Well least not a full cab of them, maybe mixing with other speakers like a lot of folks do.
I run 50watter heads into 300 and 400w cabs, so there's almost no risk of blowing the speakers. I have a 350w head but it's into a 400w cab as well