I feel like I'm very bad at playing Alchemist, and knowing when to go for it. I've been beaten by opponents who went for it when there didn't seem to be a good payload and I've been beaten going for it when I saw what I thought was good payload. What makes you want to go for alchemist and what makes you want to ignore it?
It would seem counter-intuitive, but I think the uncontested alchemist stack actually shines most when the payload is very weak (think, say, woodcutter or militia, herbalist and margrave are better respectively). In that case, a big engine will be extremely slow to get running, and so the alchemist stack with some big treasures would actually outpace it, whereas the money will either be slowed down enough by the attack or get out-powered by the +buy.
Thing is, +2 cards is draw, but not a lot. An alchemist stack also usually requires a little bit of trashing or good filter, so that is something to watch for. Even trade route gets enough of a job done.
With fast junking alchemist tends to not be worth it if you cannot clean up afterwards for the same reason as the above point.
Also sometimes there is a nice village/payload but it needs draw. +2 cards is draw. Even more convenient when it is non-terminal and always present. Of course same requirements apply as to the stack.
Ignoring is when you can get a village/draw engine up faster (gainers or really strong payload) than the alchemists and with consistency (heavy trashing). But +2 cards is also draw. Even more convenient when it is non-terminal and always present.