Not only disk space, you are limited to just two nodes (one primary and one standby). Of course you can create read replica, but that if you are write intensive, you can end up consuming all of your pools.
actually you would shard than, which of course is possible with rds aswell. since you mostly shard at the application level. there are some users that use extensions but I guess calling the support of aws will get these extension in your rds instances.
