The uncertainty attached with it!
In my point of view, action in marketing can never be successfully predicted. This visual can also be explained by saying that “marketing is (a+b)c” wherein,
- a= all the efforts a marketers takes to understand & connect with the consumers
- b= the product or service
- c= to the reaction of consumers which is absolutely variable.
Digital whiteboard 
There are unaccounted variables in any real life situation that make a consistently successful prediction impossible.
The real challenge here is to manage the risk associated and pre-testing your marketing action will help reduce the risk by making the prediction more accurate.