JMU Cloud ShareI have discussion boards and all my regular apps in the modern team and Team sites. Communication sites are more limited because they are basically publishing sites for one-way communication. You're right, you cannot put an issue app ON the page in a web part. Those web parts are coming - I saw them on the O365 roadmap today. In the meantime, I use a link from the page to those lists.
And why not switch to Yammer for discussion? Or Microsoft Teams for a smaller group?
It took me a long while to convince some of my colleagues to give up the classic pages. But there is SO much more that you can do now with modern pages that the few missing parts don't outweigh the major gains. Just imagine building a page that looks just equally good on a 21" monitor and the tiny crappy projector in your worst conference room, let alone a phone.
Lastly, you don't HAVE to migrate all your content from your traditional sites to entirely new modern sites to take advantage of all the new features unless you want to take advantage of O365 groups and hub sites. All you have to do is enable the modern site pages on your traditional sites and they will work 100% like a modern team site, except for the O365 group stuff.
Seriously, try it. I can't imagine it's that critical to have those things ON the page that you're willing to pass up all this other GREAT functionality. I mean really Sticky Headers vs discussion board or an issue app ON the page? Is that even a competition?