Multi-Wound models in general are a real pain for Custodes since your infantry is mostly stuck with D3 melee weapons or low strength / AP Guns, and T5 W3 units becoming more widespread will certainly make things even worse. The Arachnus Storm cannon may also prove a very useful weapon to deal with a bunch of new 3W models Astartes get with their 9 Edition codex. These still provide something you desperately need because, as already state, Custodes do have a lot of range anti-Armor.
Range was seriously nerfed but latest rules, making Arachnus Storm cannon less efficient as long-range anti-horde weapon unless you can deep-strike your Telemon in a safe spot where no melee unit will tarpit it. 2 Arachnus Storm cannons make them great against vehicles and light infantry. Great if they're deep striking in with your Allarus Custodians, if they don't make their 9-inch charge at least they'll have some durability against enemy shooting next turn.
Whether you take this or 6 + 1 attack depends on how you're running your Custodes, jetbikes, and Spear-wielding units will benefit from-1 to hit against shooting attacks most, while that squad of 10 Storm Shield bananas with swords, 5-man warden squad with Shield-Captain, or just any Allarus squad will annihilate anything with that + 1 attack. Consider this popular build for smashing and fuckering: Captain-Commander Smashfucker for 30 points, faaabulous Vexilla Magnifica forces opponent to subtract 1 from all hit rolls in the shooting phase for attacks targeting Custodes units within 6 of Vexilla. His wargear options are limited to Interceptor lance and misericordia, but who care? You will hit harder than any Castellan axe on charge and you can take Auric Aquila relic for 3 + save while looking so goddamn metal. Shield-Captain on Dawneagle Jetbike: This Pimpstodes is Shield-Captain with usual + 1 Wounds and Toughness bonuses and 14 move from Dawneagle Jetbike. You lose the Sentinel blade / Storm Shield option, but Eagle's Eye can fill in for it without making you give up any offensive capability. Same stat line with extra wound, but you get signature Balistus grenade launcher and From Golden Light special rule from Allarus Terminators. Shield-Captain in Allarus Terminator Armor: fat TANKY fat THICC version of previous guy. This is current 9 Edition's Adeptus Custodes / Sisters of Silence Tactics. If you're looking for Tactics for Emprah's BFFs for big daddy problem clusterfuck, you can find them here.