Hey, Im the CTO & Co-founder of ExtenduS. The product is multi-tenant, your users are isolated from one to another and from your SaaS product.

I don't understand. Say, I'm a user, I want to implement an extension to reply another user's comment automatically. To do that, I need to access the database of user comments. And then I mistakenly remove all comments from the other user?

At the moment users can only customize their own account. In the future a user can install an app in their account that was build by a developer, but the app will only have access to the user account.