Dreamers are the weakest villager role with the lowest usability in the game, second to knight, maybe.
A truthful Dreamer will tell you an evil’s true identity. If you targeted a villager, the Dreamer will tell you a random evil which is pretty much a waste of them.
If lying, the dreamer will always lie about an evil’s true identity, and tell you a random evil if you picked a villager.
I am on Ascension 130 and have a 100 village saved streak on my main account, which means I’ve played a thousand rounds or more of demon bluff. From that perspective, I will say that dreamers are very rarely useful.
A lot of the time, you mostly want them to identify as truthful or liars, and you can’t do that for sure until you have killed an evil or there is a wretch in play. That makes them a worse sort of confessor.
Sometimes with a certain board configuration, they have a percent chance of validating another character by naming someone as a demon that’s already dead when you know the dreamer is not corrupt.
They are most useful in Lilis games where their ability recharges at night, so you might be able to validate them and then get a use out of them.
If the dreamer is good in a one pooka game, they will always call themselves the pooka no matter if they are lying or corrupt. If a pooka, the dreamer will name an evil that does not exist.