Hmm well I was using a master's alembic; I have all the apparati at master's level, actually, and my potions are so powerful! Better than that rubbish you buy!
What sort of undesirable effect? Would they not cancel out then? Why not? And if one of my ingredients has a negative effect, do you still need two ingredients that have that negative effect, or does just one do it? Like in my strength potion, I had one ingredient that fortified and restored strength, one that restored strength plus some other positive thing, and one that fortified strength and drained luck. Now obviously those ingredients did other things that I don't know about (alchemy only 30), so did one of them drain luck but I just can't tell or do you only need one ingredient for a negative effect?