Regarding the teaching workload: This is not generalizable; during my undergraduate studies a significant fraction (maybe the majority? too long ago to be sure) of my classes were taught by graduate students, especially the math and computer science classes. At the graduate level, your statement was true for me at my second university. In fact, I'm not sure if a graduate student would be
allowed even to teach a graduate-level class, considering their credentials.
My experience around universities (as an academic) is that, generally, the number of adjuncts scales linearly with overall funding/skill at grantsmanship in the department. That is, the smaller universities I know saddled professors and their graduate students with substantially more non-research work, including teaching and administration.