A couple of clients are insisting on the Modern site’s approach for Project and Task management project – here are some additional notes and articles that might be helpful for any consideration.
Customization story(Still in development are Site Design/Scripts approaches) features are available to First release customers
Yes Modern is the future – It still lacks many important features/stability and still not ready for Prime Time to replace Classic in every use case. Here are some recent quote on this from MSFT and a blog on this – https://dev.office.com/blogs/update-on-modern-document-libraries-and-extensiblity
There are no plans to remove classic mode anytime soon. For more details, see our post on modern extensible at http://dev.office.com/blogs/update-on-modern-document-libraries-and-extensiblity
classic document library experience available to all customers through a simple opt-out experience, even as we roll the modern experience out globally.