Just as Leman Russ tanks are not the most competitive choices in other armies, but have a real place in the Genestealer Cult codex because of the unique benefits and synergy they have with the “horde/light infantry” style assault army that defines this force, so I think allied Tyranid Carnifexes have a new lease on life when used in conjunction with a Genestealer Cult base army.
Here are some of my thoughts on this combination:
 Massed Grav Weapons — These, more than anything else, are the bane of Carnifexes (and pretty much all MCs in the game right now) and are one of the main reasons, along with their limited mobility, that they struggle so much as a competitive unit right now. Enter the Genestealer Cult, with its unique and amazing “Cult Ambush” rules. With just a few lucky rolls of 6 for the Cult Ambush table (and every non-vehicle unit in the Genestealer Cult army has this rule, so you are going to roll on this table many times), you can have basically a guaranteed first turn charge on enemy units anywhere on the board. Combine this with the ability to take numerous models per squad and basically universal 5+ armor saves (which against Grav Weapons is a great benefit, especially when trying to survive mass Grav overwatch shooting) and you have an exceptional set of tools for neutralizing all those Grav Centurion and Grav Devastator squads by tying them up in assault as soon as the game begins. Even if you do not destroy them in combat right away, your Genestealer Cult units can prevent them from shooting for a significant portion of the game, which is key. This, in turn, buys time for your Carnifexes (and other Tyranid MCs, for that matter) to get safely upfield and into enemy lines, where they belong, and where they can wreak great terror and destruction! 😉 This alone goes a long way toward making Carnifexes much more effective on the battlefield.
 T6 Bullet Sponges — While Genestealer Cult units are generally quite solid in combat, thanks to access to massed Rending attacks, they generally are very fragile against both shooting and enemy close combat attacks, being as a rule T3. 1W, and only having a 5+ save (Aberrants are a bit of an exception to this rule). This is where a Carnifex (or an entire Brood of three of them, for less than the points cost of an Imperial Knight) comes into its own. While only moving 6″ + D6″ run each turn, their threat value, with D3 S9 Hammer of Wrath attacks and 5 x S10 AP2 Armourbane attacks on the charge (with the Crushing Claw upgrade), means that enemy forces almost have no choice but to try and bring them down before they can get into close combat. Being T6, with 4W and a 3+ save, this means that outside of Grav/Plasma/Destroyer weapon shooting, your opponent will have to dedicate very serious portions of his firepower in trying to deal with these threats (i.e. it takes an average of 72 x BS4 Bolter shots to kill a single Carnifex)… firepower that is no longer directed at the fragile Genestealer Cult units, allowing them time to dash around the board and get into assault, where they also belong. Put another way, adding one or more “Distraction Carnifexes” (yes, literally, in this instance 😉 ) to your Genestealer Cult army will absorb very significant attention and combat power, making the whole army more survivable and effective as a result. In terms of the Carnifex broods themselves, you can run them on foot in squads of 1-3, or you can take lone Carnifexes (up to 3 in a CAD) and drop them behind enemy lines in Tyrannocyte pods, bringing them into enemy lines on turn 2+ with minimal danger from shooting.
 Psychic Power Synergies — Carnifex broods benefit from Hive Mind powers like Onslaught (allowing them to run and shoot in the same turn, if they have Monstrous Bio Cannons) and Catalyst (FNP makes them even tougher to kill), but they also benefit from Brood Mind, Biomancy, and Telepathy powers available to Genestealer Cult Psykers. For example, if you cast Enfeeble (from Biomancy) on an enemy T6 unit (like a Stormsurge, Dreadknight, or Riptide) and then charge it with a Carnifex with the Crushing Claw upgrade, then the S10 attacks by the Tyranid Monster will cause ID to that target unit, killing it outright with one Unsaved Wound (or at least inflicting multiple wounds per blow, in the case of the GMC Stormsurge). Similarly, Maledictions like Hallucination (from Telepathy) and Mass Hypnosis (the Primaris of the Broodmind discipline) make a target unit much weaker against a charging Carnifex Brood, allowing them to deal with even very dangerous opponents like Wulfen, TWC, Wraithknights, Necron Wraiths, etc. Finally, the Broodmind ability “Telepathic Summons” combines very well with the threat of a Carnifex charging, bringing in squads of free Genestealer Cult units via Cult Ambush that can tie up enemy forces, pile in during close combat, or even distract enemy attention from the advancing Tyranid Monstrous Creatures.
 Mutual “Shield Walls” — Since Genestealer Cult forces lack any MCs of their own, relatively cheap Carnifex Broods can make an excellent “mobile wall” which their advances infantry and vehicles can advance behind, allowing them to get up field and into combat more efficiently. Alternately, since Tyranids lack vehicles themselves, friendly Genestealer Cult Goliaths, Sentinels, and Leman Russes can actually provide “mobile cover” for the advancing Carnifexes, giving them a cover save to enhance their survivability against AP3/2/1 shooting (this works especially well when combined with a nearby Venomthrope, since the 5+ cover from Genestealer Cult vehicles combined with the Shrouded USR from the Venomthrope to give the Carnifexes a 3+ cover save). Actually, I think a mixed battle line of advancing Leman Russ tanks and Carnifexes is very durable, very serious threat that many armies will have trouble trying to counter, especially since this combination provides threats both at range and in assault.