that's one of the things i'll give Nexo Knights. It had good ideas but it didn't do them well enough. especially when the monsters starts to just come off as being rather weak as the series continues. The fact that the Lava monsters continued to live on after Monstrox no longer was trapped in his book form and turned good might have sounded great, but personally i would have liked them to do more with the idea. Heck, just imagine if one of them like say Magmar, Lavaria or Moltor decided to become a knight. That could have been an interesting story idea.
Another problem is the part of Jestro turning good again at the end of the second season only to become evil again for season 3. Personally he should have stayed evil from the first year to the second year.
Another thing i will give Nexo Knight credit for is the idea of the Tech Infected army, which is an idea i really like. Though i hope the idea for them is that they are infected citizens of Knighton rather than just monster made from technology. It's way better when you're evil villain brainwashes the people and turns them into their own forces, thus preventing the heroes from doing too much harm on them out of fear of killing innocent people who are forced to attack them against their own will. It even have a great story opportunity by possibly having Ava, Robin, King and Queen Halbert and the other characters we have seen in the show, all get turned into cyber monsters under Monstrox's control.