i think it has a lot of potential as a learning and prototyping tool, plus you get to play around with pretty advanced graphics stuff, which is really neat. however:
The downside is that you can never really "own" your creations in a game like Dreams. They only exist within the game itself, and are only really accessible to other people who own and play the game "Dreams." You can't bundle up your Dreams creation and sell it.
this is a pretty huge downside actually. not just because you can't sell your creations, but because you're entirely at the mercy of a proprietary online service which will probably shut down in a few years, especially seeing as it's on console. also your potential audience is limited to people who also own a ps4 as well as dreams. this means that no matter how ambitious your project is, it will be ghettoized within dreams, which kinda seems like a waste.