Or maybe it is just the fact that different mixes require different steep methods.
You're as close to the truth as any of us are probably going to get. Different recipes will benefit/suffer/remain neutral to different treatment. And, this can change even more based on how we apply these methods. Time, intensity, and environment can all affect the outcome as well.
Heat is going to do several things. As you said; it accelerates molecular activity. This means the natural blending and bonding that takes place will happen faster. It makes the fluid less viscus so it becomes easier to mechanically blend (shake/stir/vibrate), and it increases the temperature which causes volatile compounds to evaporate more readily. This last one is the double-edged sword.
The evaporation can be a good thing when it is getting rid of things we don't like/want in our juices like alcohol and other volatile compounds that create harsh, off-flavored, chemical-like tastes and aftertastes. But, at the same time we are losing wanted volatile compounds like aromatics which
can cause a sugnificant loss of flavor. This can be easily seen in a lot of fruit and floral flavors. You mentioned noticing this in your own experience.
This effect can also be increased, or decreased, depending on how long you leave it exposed to the heat (time), how high the heat is (intensity), and type of container (open/closed top, plastic or glass, and how much headspace if the container is closed).
I personally would recommend anyone experimenting with a new speed steep method to start with a known recipe, one you have mixed before and know you have to your liking, then mix up a batch. Vape a little bit to get a known starting point and make a few notes for yourself. Take that fresh mix and divide it in half. Put each half in an identical container. Treat one with the new method you wish to explore. Park the other in a dark drawer, cupboard, closet, or ??? (just remember to keep it away from the pets, kids, and roommates that want to pilfer your awesome juice
).
Right after you have finished your first treatment, taste/compare both again, and add your thoughts to your notes. Repeat treatment as you desire over whatever time period you see fit; always taste comparing the two to note any differences. Once you are done with however many treatments you want (1, 2, 12 it's all up to you) store both side by side and taste every couple of days. See how they mature and which you prefer.
Remember different recipes will behave differently. A fruit recipe is not going to respond to a speed steep the same way a tobacco recipe will. Also, don't forget, this is subjective, not objective. Just because 100, or 1,000, or 10,000 people on ECF say this is the cat's meow; doesn't necessarily mean the method is going to be a winner for you. They're not wrong. You're not wrong. It just wasn't a good fit for you. Juice is cheap. Try something else.
Don't forget to play with your variables. "My doughnut recipe was a bust at 140F for an hour, but it was gold at 90F for 20 minutes." Or "My Raspberry Cream was a flop in plastic, in the UC for an hour at room temp for an hour but was a home run in glass, at room temp, in the UC for 40 minutes."
Also, it may not be the method, it may be the recipe you chose was not a good fit. Try several different styles of recipes (don't do all fruits, or all bakeries... that is unless that is all you intend to vape). "My fruit benefited from the UC but suffered from the crock pot." "My tobacco did nothing in the UC but really bloomed in an overnight soak in the thermos."
Clear as mud?
