There are a few restrictions on what add-ons can do:
Change features in G Suite
The Add-ons framework is designed to enhance G Suite applications—not to add limits. Consequently, you can't alter existing features or lock down the G Suite document sharing model.
Charge users to install
We don't provide a way to charge users for add-ons, and add-ons can't include ads. However, you can roll your own payment system or call into an existing billing database.
Detect many events
Except for certain triggers, add-ons can't tell what a user does outside the add-on itself. For example, you can't detect when the user clicks on the Google Docs toolbar. It is possible to poll for changes in a file's contents from a sidebar's client-side code, although you'll always have a slight delay.
Use the Apps Script libraries feature
Including Apps Script projects as libraries in your add-on can cause your add-on to run more slowly. You can avoid this latency by copying the library code into a new file in your add-on script project.
If you do not have read-access to the library code, you can still include in your add-on as a library. If you do this, limit the number and frequnecy of calls to those libraries as much as possible to avoid unnecessary delays.