Web applications where the functionality paid for is added as a widget to your site

There are some web applications that offer functionality that you might need for your site (paid for) and that is added as a widget, and i would like to know what you think about this?

I mean the type where you choose a paid plan, there might be various customisation that you can do while logged into the admin of the web app but for the functionality of what you need on your site it is then added as a widget, either by yourself or whoever deals with adding stuff to your site.

I think there is the possibility though of a widget slowing down a site, and even though a customer might be looking for what your web app does and wont mind a widget being added to their site, i worry that some customers of a web app might not know the first thing about how to add a widget or even how to give access for it to be integrated as part of the web app plan that they might choose.

My question is with the view of say if i have a web app and its offered that way.

Also if their site is designed badly, either visually, coded badly or both it might cause more problems at the point of trying to add a widget.

My reason for asking, my idea of a web app that would require the functionality to be added as a widget to their site seems less appealing to me now, when i think of problems that might arise.

And i now think that ideally a web app where they come and login and have access to what they need without something being added to their site, is probably the way to go.