I played and enjoyed Sierra games more than LucasArts games, but even I have to stare the truth in the face - the Sierra adventure games have aged much worse than the LucasArts games. So the answer to the thread topic's title is "Yes."
Unlike many others, I don't hate specifically on the Sierra games except for the aforementioned "last generation", which was truly bad. SQ6 feels so detached from the rest of the series in almost every way that it might as well star a completely different character. QFG5 is an action RPG with some adventure game elements, instead of being an adventure game with RPG stats sprinkled about like the other four QFGs. The less said about the rest of the "last generation" games, the better.
The parser generation of Sierra games has aged especially badly, mostly because of how primitive the parser is. One of the final parser commands of LSL2, for example, needs to be so specific that there are NO options for alternatives. If you use "INTO BAG" instead of "IN BAG", the game executes the wrong action and screws over the player. Time to load a saved game! The (graphical) user interface was a big difference between Sierra and LucasArts, best shown when parser-only titles from the Sierra titles were re-released as point 'n' click games with updated graphics. With the possible exception of King's Quest 1 EGA, the revamp improved all the titles involved. Sierra did release some stinkers now and again, and also games that had good concepts but horrible execution. The Colonel's Bequest is a great example, an adventure game that looks and feels like Agatha Christie herself wrote it: You're thrown into a family feud concerning an inheritance and a string of murders. You can question people, investigate clues...but what is the core element of the game, what needs to be done in order to solve the mystery? Being in the right place at the right time. How do you find out where to be and at what time? Via trial and error. If there's one game that could have used Braid's "rewind-time" gimmick, Laura Bow's first outing would be it.
The thing is...the LucasArts catalog isn't a field of roses either. The first three Monkey Island games, Loom, DoTT and Indy 4 are true masterpieces, but the rest don't fare so well in comparison. Sam & Max Hit the Road had overly complex and abstract puzzles, years ahead of games like The Longest Journey and Gabriel Knight 3, not to mention a style of humour that wasn't that well-received outside of the US. Full Throttle is little more than an interactive movie (though an enjoyable one), but when we get to the "last generation" era of Sierra the LucasArts titles from those years tank just as badly as the Sierra ones. Monkey Island 4 is an abomination, and I'm currently struggling through Grim Fandango for the second time. It just doesn't appeal to me. The whole Petrified Forest section can be skipped and should have been skipped (you were wrong, Tim!) and several other sections are deliberately bloated. It's just as much of a chore for me to play this game now as it was back in 2002. I'm not saying Grim Fandango is bad, but it's certainly different - different enough that it alienates some players that were used to LucasArts games having a certain appeal to them.
And that last bit is probably the factor that best describes the difference between Sierra and LucasArts. Sierra was not afraid to make and release games that only garnered to a niche crowd within a niche crowd. King's Quest is aimed at families, Space Quest is aimed at the nerds, Police Quest targets the pedantic/OCDs/autists, Larry goes for the "adults", Mixed-Up Mother Goose and The Black Cauldron is for the children, and so on. (Don't ask me about Codename: Iceman, I have no idea what they were thinking there.) It's a business strategy that certainly was viable enough to make Sierra On-Line into a major developer/publisher in the 1980s, but it wasn't enough to keep them as one.
LucasArts, on the other hand, went for a softer, more "inncloosive" approach, appealing to a broader audience and a lower common denominator at a time where gamers were becoming a bigger demographic - an approach that has a greater chance of success, but doesn't always work out. Adventure games still died, despite LucasArts's string of hits and Sierra's established fan-base of close to 15 years.