The main Problem of the Eviscerator is not enemies with Power weapons but the Eviscerator itself. Back in Dark Heresy, this weapon was completely overpowered, inflicting 1d10+10+Sb R dmg with a Pen of 5, with no additional penalties but the possiblity to hit yourself with it, which however was a neglectible factor - Good thing they nerfed it quite alot.
First problem is that its a two-handed weapon, so you cant quickdraw it, wasting half an action just to get it ready. This could of course be countered by keeping it readied the whole time, but keeping a 2 meter long Chainsaw running all the time is something worth of getting *BLAMMED* by the nearest Commissar for being both dangerous and annyoing. Aside from that, 15 Kg is really heavy for a melee weapon, limitng the other stuff you can carry round.
Second, its unwieldy, therefore you can't make lightning attacks with it or use it to parry anything. This aside, I would personally argue that due to the sheer size of the thing you simply cant use it effectively in cramped enviroments, such as the interior of most non-palace-sized buildings, inside vehicles, or in a tight infantry formation.
Third, the Pen of 9 might seem nice, but is ultimately kinda ambivalent. Its so high that it cracks any kind of non-powered armor with ease, but its too low to make it an effective AV weapon. Razor Sharp is nice, but scoring 3 degrees of success is generally rather uncommon. It could be a really nice melee can opener, but it eventually isnt quite.
Fourth, the damage ultimately done by it fluctuates alot due to it having no fixed damage bonus aside from the Sb. Its like a frag grenade, only it doesnt explode.
Anyways, what I mean to say is that the Eviscerator is a really unstable weapon that requires a lot of luck in order to be a good weapon. At first glance it might look like the Mother of all Swords, but I ultimately find it underperforming and highly wanting. At low Exp level it is still a reasonable choice, even if the weight, size, noise and slow attack speed (No lightning attacks) are then already factors that make it a choice I would think twice about. Especially given that the Eviscerator has the same rarity as Power weapons, I would always prefer one of those due to their flexibility. But as mentioned before, this is just my opinion and you may or may not that it into consideration.