Yes, there may be some disadvantages depending upon the type of free connection you are getting. For example, if you are using public wifi, there is very limited time through which you can surf it since you cannot spend a whole day at a cafe or a restaurant.
Secondly, if you are using someone else’s wifi their network connectivity issues might affect you too.
So, the moral of the story is that you may face some limitations while accessing free wifi, regardless of the source it’s coming from. However, considering it’s free, these limitations are not.