Been a while

For the HQ issue, yeah, that's a problem, but I've long resigned myself to using our native HQs in 8th edition as nothing but minor distractions and walking Ravager buffs. It's problematic to comfortably fit them into either of our transports, it's problematic to try and use them for combat because they're so much weaker than enemy HQs (do not try to fight a Smash Captain with an Archon or Succubus). Not sticking an Archon in a Venom isn't an issue when all I want to do with the Archon anyway is soak up some fire with his 2++ and give some Dissie Cannons rerolls to hit.

Their firepower isn't amazing, but it's very consistent and they throw out a lot of shots relative to the rest of our army. Three Venoms can burn through infantry at a fine rate if they're focusing fire (and especially if they're all packing Kabalite squads as well). I just see poison weaponry as... fine. Against a lot of opponents it's the same as if we had S3 or 4 weaponry. Against some it's worse, against some it's better. As long as you don't run nothing but poison weaponry it's fine.

As for whether they're fragile; yes and no. Against certain armies I find that they can weather a huge amount of punishment - using Lightning-Fast Reactions on a Venom that a Guard or Tau player really wants to kill will enable it to survive far more than you might think thanks to the -2 to hit. If they get caught in melee or shot by an automatically-hitting weapon they can disintegrate very quickly. But so what? If they're carrying Kabalites all that you've lost is part of one of your dirt cheap Troops units. If a Venom dies, it's not a big deal. If it doesn't, great. It's pretty common for most of my Venoms to be destroyed at the end of the game, depending on how things have gone, but there are usually one or two survivors.

I think it's also worth pointing out here that, even though it's not unusual for us, a vehicle getting an invulnerable save at all is really good compared to a lot of other armies! Stuff that would rip right through a Leman Russ or a Wave Serpent can easily bounce harmlessly off the Venom's Night Shield.

Speed: I run them exclusively as Flayed Skull and that absolutely helps them here. A 19 inch movement speed on a basic transport is really good! I've literally won games because the mobility of a Venom enabled me to comfortably grab an objective or linebreaker at the last moment.

Flayed Skull in general is what really catapults them from "solid unit" to "holy crap" for me, honestly. Being able to get the Archon's reroll buff with the equivalent of -1 AP while zooming around the table harassing units feels really strong! It helps that I run almost exclusively ranged armies (with the exception of, like, one unit of Harlequins that I mostly use for the Fusion Pistols anyways) - I've tried using Venoms to transport melee units before and I never feel like it's an effective usage of them, or that our non-Coven melee units are worth taking.

So, I dunno. That's my experience with Venoms; run them as Flayed Skull, stick 5 Kabalites with a Blaster and PGL in them and you've got a 125 point Troop choice that's highly mobile, a potential threat to lots of different unit types, spammable, and in the right situation surprisingly durable.

Regarding our HQs, I don't disagree with your assessment but the issue for me is that I just have so little interest in using my HQs as nothing more than Ravager buffs. If I want HQs that do nothing but sit on the backline buffing units, I'll just play my Imperial Guard.

As I said, I've thus far found Venom firepower to be less than impressive, and poison in general to be pitiful against almost every target. Nevertheless, in future I'll try concentrating my Venoms' firepower a bit more and see if that makes a difference. If it helps, I'll let you know.  

I've also had the opposite experience with regard to durability. I think the only armies that struggled to kill my Venoms were a couple of ultramarine lists (one was reliant on relatively few Lascannons, with another I managed to take out most of his anti-tank stuff early on). Every other army seemed to have no end of high strength, high RoF, multiple-damage weapons. And a 5++ doesn't really matter when you're hit by about 10 shots that do 2/3/d3 damage apiece.

I agree that losing one Venom full of Kabalites isn't really an issue. The issue is that *all* my Venoms are dead by turn 2. In fact, in my last game against Tau, all my Venoms were dead on turn 1.

Regarding speed, I don't use Flayed Skull, so I'm going by the standard speed of Venoms. Given that exchanging armour for speed is the mode for all DE vehicles (not just Flayed Skull ones), I don't think it's unreasonable to judge them by the standard.

I typically use Poison Tongue (I like the artefact and they're closer to the flavour I'm looking for for my army), so I wouldn't know. I'll admit that I'm wary of Flayed Skull because their bonus only lasts as long as their transports and, as above, my transports are almost always dead from turn 3 onwards.

However, I'll give Flayed Skull a go sometime and see if it makes my Venoms noticeably more effective. Once again, I'll let you know if it goes well.

I generally use my Venoms the same, but as Poison Tongue and minus the PGL. I also have one Venom with a non-Warlord Archon, 3 Lhamaeans, and a Succubus (if I'm using one). Basically, I just send it after a tough, non-vehicle target and hope for the best.