We hope that with this book we've been open and clear about what SharePoint developers need to learn and understand about the SharePoint Framework. For now, SPFx is by no means a perfect framework, but it's rapidly becoming one.
With FTC, the App Model, custom JavaScript's inject on pages, and other older approaches, we tended to have a monolithic and slow-moving framework that did not update frequently enough.
With SPFx, we note a combination of community work, agile development models, and lightweight tooling that quickly create customizations for SharePoint.
As such, our recommendation for developers is to keep using the SharePoint Framework, as it evolves based on customer needs, requests, and feedback. If you're fully rooted in FTC code, do not worry, as FTC has been around for a long time. However, ensure that you put some time aside to start using SPFx as best as possible.
Ensure that you start using Visual Studio Code, even...