1. The battery thing depends on fluff, actually there are models with unlimited energy supply. But everything else you say here does not add to the discussion of a moritat can use it. I was talking about the least "bladed" power weapons, most of us know that hammers/fists work different but they are nothing to discuss if we are talking about the moritat.
2. My argument was not about how wide-spread it is, my argument was about how force fields work and this weapon is a quite good example for this, no matter how often you will encounter it. And if we go in deep here, every power weapon is only asignet to elite fighters, even a "common craftmanship" version is something special and valuable.
3. The Ascention Career is not made for moritat alone, it is made for death cultists and just offers these weapons, it does not force you to use it. A death cult that has sworn to use primitive, edged weapons has no use for this kind of armament, simple as that. A power weapon is a physical weapon, indeed, but its primary damage does not come from pure force, but from advanced technology. Where a moritat uses a bladed weapon to inflict harm he uses cuting methods that leave big and bloody wounds, that is what makes him so special. A power blade on the other hand simpley burnd everything of. No matter of a moritat uses his special skills or another experienced fighter uses it. Just look at the damage code, a power weapon inflicts energy damage while a regular blade uses rending damage. This is the damage that fluffwise cautarizes wounds, just like Lasguns or for example gaunt when he cut of Larks leg with his sword and imideatly cautarized the wound with it. Does that sound like the bloody buisness a moritat is for? Well at least it does not for me. Every wound a moritat inflicts is made by a surgeon that knows how to devastate a living body. When he cuts your stomage he does it that way that your organs will fall out, he is an artist and just like you cant replace the brush of a dedicated painter with a spray can you cant replace an edged weapon with a power blade. In the end they do roughly the same but the dedicated painter can only use his skills with a brush though he might have learned to use a spray can, it is still something different.
And no, a power sword is nost just a sword with an enhancement. It is something complelty different. This weapon has been build from scrap to be a power weapon, you cant just attach something and say."yea, power field!". But more important in this discussion it is also used differently. If you just look at it with the definition weapon, it might be comparable, but if we go more into detail, as we have to, and talk about an actual blade and the physic mechanics of cuting, then the power weapon works complelty different. The actual blade you see is just traditional and cosmetic, in some models it only adds some weight for slashing power or to cunduit the actual power field. For example Gregor Eisenhorn used a power sword that had no blade at all and was just a hilt (Do not confuse this with the sollex blade).
Also it is true that a moritat sacrifices some tools and weapons and might be even weak in comparision. But this is no ****** MMO where every class has to be balanced, this is a PnP. And if a zealot takes a decision out of fath then it is that way, even if it might cripple him, this is the very essence of faith. It adds to the charakter of a PC and you have to forget about DMG comperability etc. He is an assassin and not made to fight toe to toe with enemys in power armor. As an assassin he sneaks in and kills those guys when their helemts or even complet armors are off. Yes, it might not be that easy but then you should not have rolled a moritat assassin. Everyone can read about them, it is no wall you hit unprepared. It is a choice you take and if your charakter notices INPLAY that his believes are wrong for he cannot slay the emperors enemies he might reconsider his believes. But this is an active choice. To make it more convinient by allowing power weapons is just weak cheating that has no place in charakter play, at least for my group an me. If all breaks loose, he has to b reak with his death cult, the moriat if he thinks they are wrong when they only use "primitve" weapons. That is his freedom but he has to deal with the consequences. He took a moritat, he play a moritat. To cry in the lategame for better weapons only shows that he onyl took it becauce of min-maxing, not because of the thrill of playing such a dedicated silet killer with its own code of honor.
And you said it already, there are choices like Lathe Blades he can try to acquire, also he can use called attack to unprotected areas like the face etc. Not every armour covers the whole body and is a sealed power armor. And if you are constantly fighting CSM then DH is propably not the right setting. Carapce is even managble with a Mono-Great-Weapon.
And to your last paragraph, the area where he turns the blade has already been burned out by the powerfield before. If you know ask why he stabs even futher after takin it of and then twisting it you might address FFG directly. In the end this is a PnP system that has to focus and drop rules that might never apply.
Also mostly the argument that a moritat is underpowered includes armors like carapace and even power. I just wonder where your assassins are fighting? If the campaing includes gun-ho random encounters on the street with full equiped kill teams I agree that an assassin might be in trouble here, a guardsman would be the more fitting melee fighter here. But if we realy break down what it means to be a melee assassin we come to talk about stealth and infiltration where the assassin actualy is good at. The moritat is the best silent killer in the setting for he can use silent weapons (no chain or power, just primitive) with the strongest effect. Actualy he is the only assassin career that can more or less guarantee a silent kill in one ambush. This is where his strengh lies, but most people dont get it when they compare him to auto-cannons and what not.