I think Venomancer is a hero that can be approached from a couple of different ways. The most obvious one is playing Venomancer in the support role, which is what I think most people are familiar with. The big issues that Venomancer suffer from, is that he has an attack range which is average at best, poor base movement speed and outside of his Agility growth, his stat gains are subpar. Another glaring problem is that the hero has no hard disables.
As such the hero falls into this weird kinda middle-ground between viable support and viable carry.
Now of course you can swing the odds in your favour by picking Venomancer in the right game, against the right heroes. Despite Venomous Gale having undergone significant changes in the past 1-2 years of Dota patches, it is still an incredibly strong slow. I think it might still be the most potent slow effect at level one, but I could be wrong on that. So with that in mind, the old pubstomping lanes รก la Juggernaut + Venomancer are still viable, albeit less so than in the past, because both Blade Fury and Venomous Gale have received nerfs since Dota 2 launched. The point is: Pick heroes that can make something out of the strong slow from Venomous Gale and watch as Venomancer's viability as a support skyrockets.
Then of course you can pick Venomancer to both push and/or counter-push, not to mention make life a living hell for enemy heroes that are reliant on their Blink Daggers to get good initation. Plague Wards are without question Venomancers signature ability and I really can't imagine many builds where this spell is not maxed out as early as possible. You just have to remember to get creative with it. Use it to scout, use it to block escape paths, deny runes with it and so on.
Outside of being a lane support, I think Venomancer has a place in the middle lane. However I wouldn't just idly pick him and go mid every game, as I can guarantee you that a lot of popular mids would absolutely demolish him (Invoker and OD spring to mind). But if you recognize that the enemy has picked a weak laner for mid (Pudge or maybe Shadow Fiend. Even TA, just because your dots really screw with her), then you can most likely go ahead and dominate the mid lane without too much issue.
Also with the latest rework to how Veil of Discord is built (no longer a recipe, but 2x Robe of the Magi instead) that item is honestly super dank on Venomancer. The Helm of Iron Will gives nice sustain for lane and the Robes of the Magi really help Venomancer and his shitty mana pool.
An item I would recommend AVOIDING on Venomancer is Mekansm. The item used to be a staple for the hero, but after receiving a few increases to its mana cost (Its now 225 mana to activate I think), it's really hard to make it work due to your abysmal int gain. I'd say maybe if you are mid and you're actually able to get some farm and plan on going Guardian Greaves then sure, but other than that - nah man.