The Machine levels of the Genesis Ecco games were very unforgiving. No checkpoints and deceitful auto-scrolling that often got you killed. Being a person who never liked the idea of Sonic being flattened in Metropolis Zone or Marble Zone (simply imagining it was gruesome), having Ecco suffer the same fate was unsettling.
Ecco's very fragile and there's no post-hit invincibility, so it's easy for a Vortex to grab Ecco and kill him in a second. Not to mention that in the first game, as a punishment, if you died at the final boss, you were forced to go back to the Machine. It gets easier with repetitive play, but geez.
As for the second video, Tube of Medusa is a pain, as the water is scarce and damned if you can control yourself in air. Medusa throws you around like a rag-doll most of the time unless you zoom past it. Not to mention, if you fall in the sky, you have momentum downward, so it's difficult to pull yourself up before you fall again (in some cases). Essentially, the level requires a lot of memorization. At least the music's kick-***.
Playing through both of the Ecco games was fun but hella difficult; using Sonic's Ultimate Genesis Collection on PS3, save states were available, and even though I vowed not to use them, I had to in a couple of occasions to prevent being banished to the previous level.
...I wouldn't say that the Ecco games are the HARDEST, but Ed Annunziata designs games with the mentality that only 5% of gamers will ever be able to beat them, which is ***' badass. Play Chakan: The Forever Man, and you'll know what I'm talking about.