Maybe I can give an anecdotal example that is less ambiguous. I have a fair amount of experience with psychedelics. I have had both uncomfortable trips and once what I would characterize as an actual bad trip. In the case of the bad trip it was a normal dosage, less than 2 grams of mushrooms (cubensis), and wasn't mixed with anything else. In terms of effects I would describe it as feeling like you are losing your mind, and you will never be yourself again. I imagined, in an extremely vivid and negative way, how I would be judged - by my friends, parents, society for what was happening in my head. How I would be pitied. For lack of a better word, I understood what it was to be "insane" and I knew I would never be sane again. I was unable to answer basic questions - completely unable to function (I could still speak but was highly confused). Fortunately I had friends who took care of me, changed my setting, and I came out of it after an hour or two finishing the trip feeling fine.
Psychedelics amplify your current emotions. If you feel love for the world you'll feel that 10x or 100x. If you have a panic attack, then it's the same amplification. You are correct that mixing and overdoing it are great ways to cause panic and bring on a bad trip - but bad trips really can happen if your set and setting is off. This is why people talk so much about being careful with set and setting - it is the most effective way to not play Russian roulette as you say. This is why in therapy there is always a guide - it is always in a calm place, and a therapist isn't going to recommend it for you if they see you're in the wrong mindset. But even then its not as if set and setting can somehow be measured to ensure the trip is good.
As negative as the experience was, in some ways I am glad I had it, because it gave me a much deeper understanding of where my mind could go - both positive and negative.