Been playing a lot of the Plague of Shadows campaign, which I've submitted to the database. UPDATE: entry approved.
Whereas Shovel Knight: Specter of Torment is a fully developed prequel, Plague of Shadows is a very tongue-in-cheek side-story that is happening concurrently with the original Shovel Knight. The idea is that the Plague Knight is really more of a Trickster type character, and that he doesn't care much about the Enchantress, the Order of No Quarter, Shovel Knight himself, etc. He's just off doing his own thing behind the scenes, crafting exotic concoctions in his "Explodatorium."
Give Yacht Club Games some credit: this expansion is nothing if not inventive. I think the goal was to build a set of mechanics that felt true to the character and totally unlike the main campaign of Shovel Knight. If the original game is classic platforming, this is weird platforming, to the extent that it has a major ROMhack vibe.
Plague Knight and his partner-in-mayhem, Mona
Part of that, too, is that this whole storyline takes place in the exact same levels as Shovel of Hope, only with some collectibles changed. Since those levels are designed to be a tailored fit with Shovel Knight's moveset, I think the idea here was to come up with an alternative moveset that was so weird that it could navigate any of these levels and be interesting for being unconventional. Which it is... but it's also inconsistent. Sometimes it feels cool to use Plague Knight, and sometimes it feels really against the grain. There's no smooth difficulty - there are some sections that weren't particularly tough for Shovel Knight that feel much harder for PK kind of at random.
Combat-wise there's a somewhat deep bomb-crafting system here, and the flow of play slows down to about half-speed as you figure out which tools to use as you go. It's a much thinkier take on the material.
Fun, but weird. Feels like it is occasionally difficult by accident, and never difficult at climactic moments. I'd probably rank it behind Shovel of Hope and Specter of Torment, but it's certainly not "more of the same." I can see how, coming out of this, they realized they really needed to build whole new levels for SoT.