I have done the edibles/shrooms combo a lot of times.
Cannabis can hugely amplify and deepen the effects of classic hallucinogens. And if you realy want to experience the full force of the synergy between cannabis and classic hallucinogens, then edibles are definately the way to go.
I personally realy love the synergistic effects, but many people don't. The reason being, that the synergy is very powerfull and a bit unpredictable as well.
The best way i can describe it is that psychedelics are not just amplified enormously, but also become "more psychedelic". The weirdness gets weirder, the strangeness gets stranger, confusion becomes more confusing and paranoïa becomes more paranoïd.
Euphoria becomes more euphoric as well so it's not just the challenging parts that get a boost. Colors become warmer, more saturated and brighter and visuals start flowing more. It's realy like you add lubricant to the visual system.
The combination works best with psychedelics that are a bit of a stimulant as well, like LSD or mescaline. With shrooms, as the synergy goes both ways, you can become incredibly stoned and sedated if you take too much cannabis. I even fell asleep on one occasion. The stimulant effects of LSD and mescaline balance out the strong sedative effects that you can get from cannabis, and shrooms don't do that.
I think the main reason why some people realy don't like it, is that you are definately less in controll. Or you feel like you are less in controll. I would even say that it sometimes very much feels like you are losing controll and slipping away.
This is purely because of the incredible stonedness. You can definately take too much cannabis, and then you can end up in a sort of deeply sedated half asleep/half awake state. Even on LSD.
I would say that if you're not very experienced with psychedelics, this could easily become a bit too much. It very easily overpowers you.