Tying yourself to a specific provider is dangerous.
RDS and EMR are fine, since you can reproduce the exact same API anywhere else. S3 is also fine, since its API is simple enough and even reproduced by other vendors. Something like DynamoDB I wouldn't touch.
Things like AWS are great for when you don't yet know what you need, for absorbing load peaks and for getting something running very quickly. They are however the most expensive thing you can buy for what you get.
RDS and EMR are fine, since you can reproduce the exact same API anywhere else. S3 is also fine, since its API is simple enough and even reproduced by other vendors. Something like DynamoDB I wouldn't touch.
Things like AWS are great for when you don't yet know what you need, for absorbing load peaks and for getting something running very quickly. They are however the most expensive thing you can buy for what you get.