[gorgonical]

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.

[godelski]

At both the universities I went to most classes were taught by the professors. I say most because when I was the TA for my advisor (during my PhD) I taught his class. That said, the students were happier when he didn't show up to class and it was only me.

It definitely depends on the size of the university and the size of classes. As I was graduating a few grad students started becoming the official instructor. These were only the lower level courses though (freshman and sophomore). My partner's department had grad students teaching some classes for longer and they had a similar pattern.

My undergrad was at a small university with essentially no grad students. As far as coursework, I'm confident I got a better education than my peers that went to top schools like Stanford and Berkeley (I did physics). But they got more internships, connection to labs, and connection to research projects. YMMV