I kind of look at like this, you take monkey brains, frog eyes, the blood of a wolf at a full moon, a pinch of sulfur, and rotten mushrooms and mix it all up to make a potion, not a potion of testing like the scoobs did, but lets say a strength potion.
Now you could get salmonella from the brain or the eyes, rabies or worse from the blood of the wolf, you could have a sulfa allergy or penicillin allergy that would spell game over for you (if it was severe enough) for the sulpher and mushrooms, yet mix it all together and that presto chango old world magic of alchemy (not real world chemistry) would kick in and make all those harmful ingredients A-OK and safe, that is, as long as it was made correctly. I'm sure you know fantasy is ripe with mismade potions and effects gone goofy.
I really appreciate you pointing this out and making me think about it, beacuse now I'm thinking this was probably Willows first attempt at magic. (Before this conversation my opinion was her first experiences with magic were much latter.)
We see chemistry, but I think it was intended to be accepted as old world (fantasy)alchemy.
Now would Giles approve her using dangerous chemicals?
I don't know, I can see why you could question it's ethics, she could prepare it wrong and burn Amy by a failed potion. If he thought this was simple enough for Willow to do, with Willows Chemistry background and familiarity with tools alchemy shares with chemestry...scales, calcinators, mortar & pestals etc. He may be willing to gamble the risk due to his faith in her and the ease of the potions creation.
^^^^and all that crap is just my made up supposings. Willow may not have even known there was a risk; for that matter, maybe there was none.