Sometimes, taking one blotter of a research chemical would cause no ill effects, and depending on the circumstances, one could even have a euphoric and positive experience on an RC. On the other hand, one blotter of some research chemicals would put you in the hospital or even kill you. It all depends on the specific substance, the amount that is contained on the blotter, and your individual body chemistry and the emotional state that you're in when you ingest it.
As for the substances you mentioned, some people take 25-I on purpose. I took 25-I by accident once, and I had no negative side effects. I actually had a great time with it. However, it is more dangerous than classical psychedelics. If you happen to be sensitive to it or the blotter of 25-I is dosed too strongly (e.g. a hotspot), you could end up suffering seizures and winding up in the hospital. It also causes some people to suffer severe psychotic episodes, even if they were not predisposed to mental illness. A friend of a friend of mine took 25-I, and it caused him to essentially become completely delirious, as in datura-level of delirium. This episode resulted in him getting arrested for disrobing and charging down the street with a stick. Luckily, he did completely recover from his episode after a brief period in the hospital (and even tripped again on real LSD with no problems), but others have had permanent side effects from 25-I. If you aren't sensitive to it and the blotter of 25-I isn't strong, the duration is similar to classical psychs. In my experience though, most of the time when someone takes 25-I thinking its acid, if it is just one hit, nothing disastrous happens. It can be extremely dangerous as I was saying, but it seems that the majority of people don't suffer these effects. Of course, if you happened to be one of the less lucky ones, this would be a moot point. 25-I is probably one of the most common RCs that are sold as acid, if not the most common one.
DOx drugs are potentially dangerous due to vasoconstriction. It can be simply uncomfortable but otherwise a positive experience for some people, but the vasoconstriction has also been known to cause very dangerous effects, including damage to tissues as a result of vasoconstriction. Sometimes, it can land a person in the hospital or even cause damage to appendages. Also, it lasts a very long time, sometimes as long as 30 hours. And it can take hours to peak, which sometimes results in people taking too much and suffering serious medical emergencies. Luckily, DOx isn't usually that common, but I have known people who got DOx that was sold as acid. DOx drugs are very different than acid, and it has much more of a stimulant quality as they are substituted amphetamines rather than being classical psychedelics at all. They can cause visuals and all I hear, but they are essentially stimulants with psychedelic qualities.
Bromo Dragon Fly is luckily very, very rare. However, this has to be one of the craziest drugs I've ever heard of, if not the craziest one out there. Never saw it myself, and I don't personally know anyone who has had an experience with it. It can be sold as acid, but it is extremely uncommon. The effects are similar to other phenyltamines and is fully a classical psychedelic (e.g. not amphetamine-like at all), but it lasts an agonizingly long time. People trip for up to 72 hours on this stuff. Not an afterglow sort of thing either, people actually experience a peak effect that lasts as long as 72 hours, and it is intense as fuck. The visuals are often even more intense than ones on acid. I watched a YouTube trip report about the stuff once, it was crazy. The woman who took it said that it lasted like 48 hours and she kept hallucinating the sun rising and setting over and over during the trip, you know you're tripping but the hallucinations are highly realistic and much more all-encompassing than most acid or mushroom visuals. Also, the comedown is likely quite rough as during this entire time, you obviously don't sleep at all and are very sleep deprived. If someone took Bromo Dragon Fly knowing what it was and expected a 48 hour plus trip, they could enjoy theoretically enjoy the experience of tripping balls for three days if that was the experience they were going for as the substance doesn't have any particular tendency to cause anxiety or paranoia any more than other psychedelics as far as I know. However, if it's sold as a psychedelic that lasts 8-12 hours....... and then the stuff ends up lasting for 72, it has a way of seriously freaking people out as most people don't plan on tripping for 72 hours. Also, the extremely long duration could get in the way of work and other obligations if one is not aware of what they are taking and thus is unaware that their trip is going to last for two or three days rather than 12 hours.
Also, people have ended up in the hospital from the stuff due to physical side effects. However, since it is so rare, very little is known about this. Apparently, Bromo Dragon Fly has even resulted in a few deaths. However, it's not known how common this is and the full extent of the health risks since so little is known about it due to its extreme rarity. So, all in all, Bromo Dragon Fly is probably one of the worst RCs out there given that it has the potential risk of death (even if it's actually a relatively remote risk, though no one really knows whether it's a large chance or small chance as so little is known about it) along with the fact that it lasts for up to three days.
Then again, there are some RCs that are relatively safe and not really more dangerous than acid. For instance, 1-P LSD is pretty common. That stuff is basically the same trip that you'd get from acid, perhaps even identical. Essentially, 1-P LSD is acid, just a slightly different variation. Likely wouldn't taste bitter either as far as I know, as it is essentially just a different form of LSD.