Lightning Claws seem incredibly underwhelming in comparison to a Thunder Hammer (and, given the errata for Into the Storm that has the weak model specifically designed for unaugmented humans to wield receiving a doubling of strength bonus on top of extant qualities leading me to anticipate this change for the Deathwatch model Thunder Hammer as well) and the Power Fist.
The Lightning Claw's low damage seems like the cherry on top of essentially losing the use of the hand, and the increase of the fine manipulation penalty to -20. The additional damage per degree of success does not seem attractive, and it seems almost unthinkable to dual-wield the things given what they do to a character's ability to have any degree of manual dexterity (the discount on a second Lightning Claw is nice, I admit, but I still balk at spending 45 requisition to get a pair, which, really, to gain much utility out of I must have the Two-Weapon Wielder talent). I suppose it cannot be dismissed that Lightning Claws are weapons one can employ to Parry, which is not an inconsiderable advantage for a melee focused character likely to have invested significantly in increasing Weapons Skill (for my part, however, I would take a Combat Shield from Into the Storm and take advantage of its Defensive property and added armour, or perhaps even an Astartes Combat Shield - both of which allow the use of the hand of the arm they are attached to, which to my mind makes them attractive in pairing with a Thunder Hammer or Power Fist).
Of course, it cannot be understated that my skill in mathematics is not especially strong, and that I often have difficulty in seeing the larger picture, so I come to you prepared, in good faith , to be illuminated. I fully expect that there is some combination of Talents that I have failed to consider renders this choice particularly attractive, or that when the mathematics are laid bare I will see how well the Lightning Claws actually perform in practice.