As a SharePoint developer I have traced different approaches and possibilities of the SharePoint customizations over the years. If you are my colleague, working with SharePoint, you have probably experienced different ups and downs working on SP projects, especially in the development department as the customization approaches have changed over the years and versions of the platform. So, we have built C#/.NET based web parts exclusively in the beginning, but we have also used Script Editor Web part – it was easy to customize DOM on the classic SP sites this way, but we have learned that it cannot be used on “NoScript” sites. Later we got App Parts – Developed using Add-in model, placed in an iframe. Development of it is a little bit complicated and it is not able to access DOM of SP page.