At this point it's a lot of guess on what the downsides will be. On the pages for Aurora they mention they're working on support for extensions and various PLs,

At this point it's a lot of guess on what the downsides will be. On the pages for Aurora they mention they're working on support for extensions and various PLs, but it's unclear if they're supported in the beta at this time. It's also unclear if they had to fork off in some way, if they did it will most likely lag behind what core Postgres at each new release. Of course right now it's a lot of guesswork. It should be much more clear when it gets into GA on what the trade-offs are.