I think for most people on this forum, bad trips are rare. But i don't believe that anybody can be immune for them. I once though i was immune to bad trips. But then, one day i took acid and accidentally fell on the floor. That caused a wave of negative thoughts and it became worse and worse and my mind was racing so fast that it felt that i couldn't keep pace with this 'negative-thought ferrari in fifth gear'. I think with mushrooms, DMT or ayahuasca, there might be a chance of the experience being too heavy. But it's easyer to remain calm with them. The energy of LSD that can be so euphoric, can also be a disadvantage here when it starts taking you over. It's a sort of energized fear then, that can lead to total panic. With ayahuasca i once had the scary thought that i might have eaten tyramine containing food and that i would be possibly one of those one in a million people sensitive to this tyramine stuff. But no matter how powerfull the stuff was, i could remain calm and i could tell myself that there was no need to worry and a minute later i had almost forgotten about this. With LSD it's much more difficult to remain calm at all times.
69ron told in some thread that LSH and LSA can cause shortness of breath. I think LSD has this effect as well, be it smaller. Normally you won't notice it, but when a panic attack happens it becomes very noticeable and then this is another real disadvantage about acid.