Quick note, what everyone had seemed to miss in here is that yes, Power Weapons are nigh well common for Deathwatch characters, and not excessively uncommon for members of the Imperium. However, as you are not often fighting other deathwatch members or other members of the Imperium, the question is very valid as to how common they will be for your *enemies*.
The Tyranids sure aren't going to be using power fields and sundering your weapon. One salient cleared.
The Tau, well, Kroot aren't going to, nor are Fire Warriors or really anything short of a master level opponent. So you must use some caution, but as long as you're not a fool you're ok.
The Chaos forces, it's sure not going to come up against cultists or demons. Both the high and low end will be fine, the only real sticking point is Chaos Marines themselves, and then it's the same as above- use your eyes, if they have a power sword, use caution, if not, go wild.
The rest of em, well, Necrons are a problem, Eldar are likely a problem, and Orks worth consideration are a problem.
So, ultimately, it's likely fine most of the time. It's a gross exaggeration to say that he would even encounter an enemy with a power field in every mission, likely only 25% of the time. Those times, he can either use intelligence (see the weapon, use something different), use tactics (have someone before him burn up the opponent's reactions, so they can't parry), or use luck (it's not a guarantee, after all). If it breaks on 10% of the missions, with all the backup options available, is it really that big of a deal?
The real question, of course, is why he would choose to use that weapon as his signature weapon vs the other options available, as it's mechanically equivalent to a power sword in most cases, worst in the rest of them, and strictly inferior to the better power weapons. If it's just "because it's cool", then personally, I would say screw it, and say that an Eviscerator is a chain sword with the power field, and model it after the step from claymore to power claymore in terms of damage and pen increase (leaving it at 1d10+8 pen 8, and giving it the power field, unwieldy, and tearing qualities).
Trust me, this is not game breaking at all, and is exactly in line with other options available. Note that tearing on a 1 die damage weapon is a slightly less than 2 point increase in average damage done (reflected in its stats compared to other options), and flesh render is mechanically a less than 1 point increase in average damage. If you can't handle that from a fluff standpoint (even though it seems ok to me, I know less about the fluff behind eviscerators than some of you, maybe it's spelled out that they don't have power fields somewhere), then my earlier point stands- if it breaks from time to time, that's also not game breaking.
Edit: Realized I fubar'd the math. Not by much, though. Forgot to take into account extra righteous fury. Base chance is 10%, chance w/ tearing is roughly 20% (19%), chance w/ flesh render is roughly 30% (27%). However, the extra 10% chance of 5.5 damage from those is a whopping extra .55 damage per attack, so tearing is almost exactly an increase of 2.2 damage, and flesh render is 1.37 more (for a total of 3.6, which is still with the proposed stats in line with other options, and regardless again, not game breaking).
Edited by Dr. Quinn, 15 July 2014 - 09:05 PM.