This mod is not too hard. Version 3.51 made it harder, but difficulty can still be upped especially for the end-game when the player has special noble troops. In my games, I've restricted myself to never recruiting any nobles, so the only special troops I can get are from the normal buildup from garrisons in towns that have Orders and from capturing nobles in battle. It is too easy when the player has 100+ Griffon/Dawn Knights pumped up 4 times.
With PoP, you have to respect certain enemies and not just charge in blindly swinging away. That makes battles interesting. You must anticipate and respect any enemy that can throw stuff. You want to know what the secret is? Get on their right side when chasing so that they can't throw. There's a large blind spot for throwing/bows and you should take maximum advantage of that. Before you hit that safe area, keep your shield UP!
I don't think Shadow Legion are overpowered. They can only be created from Empire Knights, so if a player is going for the culture of the Empire, then all other Pendor Knights are off-limits. No Silvermists, no KotEG, no Dawn Knights or Griffon Knights. That's because pendor nobles are very hard to capture, almost no AI lords get them in their regular army. So you can only get a bunch of Shadow Legion Knights if you sacrifice other paths which seems fair. By the way, I'm glad that the Empire Immortals have been given more of a chance to have shields. They were good before, but too vulnerable to missile fire. Now they are a nightmare to face.