I was excited to see this. Sadly though it was only a limited series (as in one series) and it only had 10 episodes, and the whole 7 episodes were wasted with the characters stuck in the past.
Plus, this character Douxie; we didn't know much about him other than his frequent appearances in Trollhunters and 3Below, and then suddenly we see his full story in this one, but it felt sudden. And that's my other critique; the whole thing was rushed.
And finally, while I am happy that Jim is human again, I felt that it spoiled the finale of Trollhunters though (I felt it was a disrespect to the late Anton Yelchin).
But those were just the cons. My pros for this were the full cast, the epic fight scene between the knights and the trolls, and the surprising origin story of the first Trollhunter.

It was pretty good though. I just wished though that the whole thing wasn't rushed, and there would've been more than one series.