Honestly not surprised. The theme had pretty good sets, but, much like Chima, suffered from an awful story. Once the Nexo Knights have beaten the tar out of Jestro with their amazing "Ultra-cool-Nexo-power-that-totally-isn't-the-same-as-that-last-one" Shields (That turns into a really annoying gimmick that doesn't reappear and just makes the consumer shy away from buying the product) the umpteenth (actually 3rd) time, it starts to get really old really quick.
And there's almost no status quo shift. Sure, Clay turns into a rock and goes evil, but so many things could be done to the plot to make it connect more fluidly than what's present. Jestro's dumb and has second thoughts again even when he's supposed to be pure evil. The King is overprotective and can't get past that. Lance is annoying, rich, and pansexual (though there's nothing wrong with pansexuality).
I'd like to highlight a particular example: in season 3, the knights lose their shields. I repeat: THEY LOSE THEIR SHIELDS. Not only would this be a way to (temporarily) stop one of the most overused deus ex machinas in the show, it's an opportunity to develop the knights' fighting ability rather than having them on their crutch against the "very powerful" ball and dumb dwarf enemies. Instead, the knights get their stuff back by the end of the episode and ruin what could've been a major turning point.
Basically, Nexo Knights is a run-of-the-mill Saturday morning cartoon. However, unlike say, Scooby Doo, Nexo Knights has neither the correct formula nor the likeable characters (and yes, they're both flat tropes. Also, ignoring Ava and Robin) to be enjoyable.