First of all, the dude in the first video was right. By no means did he "invent" that exercise. It's pretty much exactly the form Vince Gironda recommended for hack squats back in the '50s.
Sissy squats can be decent finishers if your knees can handle them. Essentially, they're bodyweight leg extensions, so if you treat them as such (using a proper warm-up beforehand, or ditching them completely if you have pre-existing knee issues), they should be fine.
Here's an article that talked all about them:
Roman chair squats (like in the third and fourth videos) were used frequently in the '60s, '70s, and '80s by guys like Arnold, Frank Zane, and Tom Platz. They'll be more gentle on the knees, since they involve more total leg musculature, but again I'd use them as "finishers" since it'd be difficult to use enough load to make them a solid first exercise on most leg days.