Web AppBuilder vs App from scratch Javascript API

[AlfredoFernandez1]

I need to do some UI modifications/customizations of an existing app built using Web App Builder. If I download the source code and open it in Visual Studio, is all this app configurable and is it possible to fully customize everything from the UI perspective?  Or is it better to start up a new project in Visual Studio using Javascript API and start from sratch? 

What is the adventage of using web app builder instead of creating an Javascript API app from scratch? If someone can provide some advice will be great.

[RebeccaStrauch__GISP]

Web AppBuilder for ArcGIS (Developer Edition) | ArcGIS for Developers gives you access to the many custom/extended widgets that the user community has written, that may or may not ever become part of the base software.  It allows you to nit "re-invent the wheel" since many time, others gave had the same needs or requirements as you.

check out Web AppBuilder Developer Edition - Customization Resource List​ for a list if many of these available widgets.  These widgets can be  plugged  into WAB and the configured. You can also take the widget itself and further customize it for you needs if necessary.  The WAB community is very active if you have questions, and you can share you widgets if you have something others may be able to use.  You should checkout and follow Web AppBuilder Custom Widgets​

[ChrisSmith7]

If it was developed using the WAB, and you're needing to customize, I would check-out the links Rebecca gave first. If you find this doesn't fit your needs, you can leverage complete control by developing using the JSAPI (| Guide | ArcGIS API for JavaScript).

I started using the JSAPI before the WAB was available for JavaScript, so I prefer this, but only because we already have a customized app already built from scratch.

[TyroneBiggums]

If you're comfortable creating an application from scratch, and you're versed in JavaScript well enough, I would skip WAB completely. That is, if you also have the time. I think you'll want to create your own if you want to really fine tune the functionality and UI. If it's just UI, I'd have to defer to Rebecca's post. But, I thought there was really only so much you could do in UI on WAB.

tl;dr? If you want limited customization and out of the box functionality, use WAB. If you want full control, build your own from scratch.