There is an incredible ammount of secret areas, hidden items and various details. And the best part is its all optional and up to you
Yeah, but for every Urshilaku Burial Caverns, there's plenty of worthless shit like this (and this is a quest dungeon):
You can say that Oblivion is decline from Morrowind, or that Skyrim is decline from Oblivion, which is true, but dev philosophy and core pillars are too different between Dagerfall and Morrowind
But the thing is, it's not just that it's a "decline" from Daggerfall, it's that it doesn't make the most of the change in dev philosophy. You absolutely
can make a game that retains many of DF's strong points while also benefitting from the opportunities offered by a smaller and more focused worldspace, but Morrowind generally isn't that game - combat is poor despite being a major focus of the game, range of viable character builds is needlessly reduced, NPCs have no routines even in the most basic sense of stores closing at night (which cripples thief classes and makes the game lose out on the sense of """immersion""" that could be offered by the smaller handcrafted world), and all the other shit we mentioned.
The question isn't just how MW compares to DF, it's also how well MW stands on its own and how much it made of the potential offered by the new format. The weaker dungeons aren't just weak because they're not like the ones in DF, they're weak because they're not very good on their own.