i don't personally find that Venom Spit is severely imbalanced (as some claim it to be), but its definitely one of the strongest abilities in the game. it is balanced out at the moment by a fairly high mana cost (and by Beasts naturally somewhat small mana pool) but nothing is sacred and everything ought to be subject to review for rebalancing if necessary. Spit is currently high enough on the power level scale that it ought to be examined.
regarding the counters the OP mentioned
Oak's Shield does counter it completely, but is on a much much longer cooldown so it doesn't really neutralize the ability, it just counters one application of it. you'll get spit on again 7 seconds later and won't be able to Shield that one.
Queen's Bramble Shield doesn't counter it at all really. The spit will damage the shield with no loss of efficiency. it may seem that a Queen is able to avoid taking damage by Bramble Shielding through it but this isn't entirely accurate, its more like Queen just uses Bramble Shield as a supplement to her hitpoints in general. Spit still eats through it.
Erebus' Mist is a legitimately efficient counter to Spit. you can stay in the Mist for just 1 pulse and only spend 125 mana to remove all debuffs. this is fine though, its quite balanced by the penalty of having to spend about 3-4 seconds shifting into and out of Mist and doing basically nothing during this time.
Sedna's Heal is a pure counter to it at rank 3 and 4 when it will actually just remove the debuff completely. At rank 1 and 2 its not really any kind of counter, it just restores some lost health but it doesn't do anything to reduce the overall damage dealt by Spit.
the favor item and consumables that remove debuffs are good counters to it and are probably the best option for most DG's, though there's obviously a fairly high opportunity cost associated with using those.