Overall, I’d say it’s about a wash for durability - scrolls don’t break when you drop them, but potions don’t go up in smoke at the first hint of an open flame. (And ACKS doesn’t have an Item Saving Throw table anyhow.)
The official table is discussed at http://greyhawkery.blogspot.com/2011/05/dmg-reflections-potion-miscibility.html
No handy link unfortunately, but OSRIC includes a “Potion Admixture” table on pg 328 of the latest.
From that one, 31%+ is a good or excellent result, which tracks with the 1E table; in play you’d probably never notice the difference. OSRIC specifically states:
Admixing a potion of delusion always makes you think the other potion is in effect, though it’s not (invisibility being the example)
Admixing clair(voy|aud) with anything stuns you for 1d4 days (sensory overload).
Admixing longevity slows you for an hour.
Polymorph always produces a strong poison.
Interestingly, perhaps germaine, (and I don’t recall if this was the case in 1E) OSRIC states that potion manufacture is a joint venture between alchemists and magic users until the magic user is 11th level.
And if I were to use that for ACKS, I’d change the 1% to be something like:
Take 6d10 damage, and roll on the Mortal Wounds table as the Dismember spell (Only suffer the result on Permanent Wounds, not Condition & Recovery; only CON modifier applies)
because a bad potion mix literally blowing off an arm or something would be hilarious.