I think its problem lies with placing a complex strategic burden on the player to your left. It's only slightly worse than "+1 action, draw to 6", and can be better if the player to your left makes a "mistake". Unlike Envoy and Advisor, where there's usually an obvious choice of which card to discard, every card you discard with this gets "cellar'd", so it's up to that player to determine whether discarding a certain card is worth it given the potential quality of a card that would be drawn in its place. Leaving behind dead cards is a no brainer, while discarding the player's only copy of a key action card is similarly simple, but there's a lot of grey areas in the middle. It does ask a lot of the player to the left's knowledge of your deck.
There wouldn't be too much AP in any given game since this card doesn't stack very well, but it's definitely an "advanced" card.
On the other hand, "+1 action, draw to 6" on its own is probably in too good for 4, not good enough for 5 territory as a guess. Then again, I assume much of the value of Lab comes from stacking.