The real issue with Mordekaiser
To start things off i must admit it I'm not a hardcore Mordekaiser main like MM, just a casual P5 player who had a hell lot of fun with Mordekaiser before his rework and haven't touched him since. I think the true problem is not his second HP bar (or shield) but something else what lays beneath the Juggernaut Class itself. So before i jump right into the Morde topic I would like to give you an insight on the Juggernauts as a class.
I think everyone knows that the Juggernauts are the ultimate solo class, they are the least team-dependent and have the best chance of solo carrying out of any class. They have huge HP bars and obliterating DMG but they are slow moving and kiteable war-machines with limited options of closing the gap or CCing the enemy. Although they are living wrecking balls they can easily be countered with kiting and CC. They represent a Feast and Famine playstyle which can be quite enjoyable, but not too healthy. If they are fed they can be unstoppable (the so called Darius-syndrome), but if they are behind they seem non-exiscent (at least until they get their 3 item powerspike). After this sort introductory let's have a look who truly belongs to this class.
I think he is the most well-known juggernaut and there's a reason for it. He is the definition of a true Juggernaut: he is slow, tough and has devastating DMG with quite good self-sustain and also his ULT DEALS TRUE DMG! So yep...
What should i say about her except that she has the highest base dmg and one of the highest AD ratios on her abilities in the game, again with incredible self-sustain and 1v5 potential. Tentacles rules... ( and also zero skins... :( )
So everyone's favorite troll, Trundle. He isn't a too traditional Juggernaut but still belongs to the class. Has great self sustain with his passive, high DMG, high HP and he is also the most mobile and has the most effective tool of CC of any other Juggernaut in the form of Frozen Domain and Frost Pillar. Also, his ult is a plain stat-buff, what else should we wish...
My favorite champion, good ol' Mori. He is one of the few minionmancers in the game and quite good at it. His Qs hit like a truck with only a Sheen, has high HP, not so superior but still good sustain on his Q and the coolest ultimate in the game (at least when the Maiden is not standing still in the middle of a teamfight...). Maybe, I'm a little bit biased...
And last but not least:
. As you can see by this time, Morde is the only AP Juggernaut in the game, and I think this causes the problem. All of the Juggernauts have insane ratios on their abilities however they are still balanced. An AD champion is not able to stack AD purely and still be effective, but an AP is capable of that. An ability with 80% AP ratio can't be balanced so Riot have to maintain Morde in this underpowered stance or face the consequences. 400 AP is easily obtainable with only 3 items, and the rest can be defensive, but that's not the case with AD. So, the final conclusion is this: Morde must remain underpowered if they want to keep him AP. Maybe I'm terrible wrong, but these are my bets. (Also, sorry for any grammar mistakes, I'm not a native English speaker)
-An ex Mordekaiser player