I just played an Alchemist game in which my opponent bought two potions too early and drew them simultaneously, which hurt his ability to buy Alchemists. So without a lot of money and +buy, two potions is going to cripple you early, when that extra potion could have been a silver or useful action letting you get Alchemists or Golems or whatever it is you need.
Even without +buy, an Ambassador/Potion opening allowed me to trim down my deck and consistently top-deck 5 alchemists, drawing $8P every turn and cleaning out anything he was giving me with his own Ambassador. It led to a rare game in which I won with 8 provinces (one on every turn from 14-22 except for turn 18, when I bought a Gold because I felt I needed to offset the green).
Gamelog, just cause I'm usually pretty awful at this game and kind of proud of it:
http://dominion.isotropic.org/gamelog/201210/24/game-20121024-195702-66362ca4.html