I think the difference between Sierra games and the others you mentioned is that it's possible to finish them without a walkthrough. And really, there's nothing in those games that entirely defies logic. They might be very hard, but they're fair within the rules they lay down, and you can't end up in a situation where the game is unfinishable.
Sierra games have a lot of "dead man walking" scenarios where you end up in a situation where an action you took hours earlier locks you out from finishing the game. They also have lots of instant death scenarios that you can only find by trial and error. Is your last save several hours old? Too bad, you get to play that part again.
Their games also feature a lot of "dream logic" puzzles where you really have no way of figuring out the intended solution short of clicking on everything. The "cat hair mustache puzzle" is a great example of this - https://en.wikipedia.org/wiki/Cat_hair_mustache_puzzle. To be fair, that one wasn't created by Roberta Williams, but I think it typifies much of the Sierra design approach.
Again, Jimmy Maher at The Digital Antiquarian has gone into a lot more depth on these issues in Sierra games (and plenty from other companies), comparing them to better designed games from companies like Infocom and others.