The simpler story is that it is better to eat whole fruit.
The problem with eating smoothies or even drinking fruit juice is that you are increasing the fruit's glycaemic index. For example, an orange has a glycaemic index of 40, while orange juice has a glycaemic index of 501. The theory goes that low glycaemic index foods lower the risk of metabolic disorders such as obesity and diabetes and there is indeed epidemiological data which confirms this specifically about fruit and fruit juice:
Greater consumption of specific whole fruits, particularly blueberries, grapes, and apples, is significantly associated with a lower risk of type 2 diabetes, whereas greater consumption of fruit juice is associated with a higher risk.
based on data from 3 studies which followed a total of ~150 000 women and ~35 000 men for 18 to 24 years.2. Note that the consumption of fruit juice increases the diabetes risk when compared to baseline, not just when compared to whole fruit.
Seeing that the average person in a Western culture has a higher chance of developing a metabolic disorder than of having difficulties digesting cellulose, we can conclude that, ceteris paribus, fruit is healthier for you than fruit juice. I did not find data on smoothies and purees, they should fall between whole fruit and fruit juice in glycaemic index.
The story gets murkier when we consider your diet as a whole. People are unlikely to eat a whole orange when they are thirsty. So if you are currently drinking orange juice and planning to switch to the same amount of whole oranges and switch to drinking water, this is probably going to be a healthy decision. But if you are going to switch to drinking Coca cola when thirsty, it will likely be worse. While I did not look for data doing this comparison, the glycaemic index of Coca cola is 631, and it also contributes many calories without them being paired with micronutrients.
There has been some debate on whether recommending diets based on GI makes sense, see these two 2002 reviews (paywalled): 3 and 4. Anecdotally, I'd say that they make sense, as I have seen people lose weight on them without complaining of hunger pangs.