A PHP MVC controller to display a form and add an item to the database
If we talk about good practice then "Better not to do much in __constructor()". 
Constructors are built to instantiate a class. They must not be overloaded with huge code. For providing variant functionalities, use methods instead. This is how to work in modules. For a specific kind of task, it is better to call the specific function instead of overloading the same method.

However, you can do so (and you are doing), but it is not a good approach.
Consider an example,

You have to perform 10 actions for a given event. You can do so in same function. But it would one day become much complex to add new functionalities in future. However if you are dividing the whole task in modules, you can and will be able to add new functionalities with no overhead. Also this makes your code:


General
Flexible
Easily understandable
Easily modifiable
Clean
Best practice


Also not all frameworks follow complete MVC but you can see in any one of them, separate functions and classes are there for specific task/action. This may cause to write more lines of code but since this is a good practice, almost all the developers follow the same mechanism.

However, it is user specific opinion. At last, you can do anything you want to do cos you are developing &amp; may be this approach is somehow good in your context. Choice is yours.