‘It was all a dream’ is already an incredibly hated plot device. It makes the viewer feel like the entire episode or story arc was completely pointless as even if the character having the dream learned something from it, nothing that happened, and none of the other character’s character development ever happened.
Speaking of the other characters though, you gotta feel sorry for them. Imagine, when the dreamer wakes, everything in the dream ends. In your own dreams, it doesn’t matter so much because you are the one waking up, but what if ‘this’ was a dream, and you weren’t the one doing the dreaming?
What if everything you remember and know is only known as part of the context of the dream, and you have really only existed for the last couple hours, and in an unknown amount of time, not only will you and everything you know cease to exist, but everything you’ve ever cared about never really existed in the first place.
It must REALLY stink for them if characters in a dream can become lucid like a person who is dreaming can.