Tasks should be unified across o365, i.e SharePoint tasks list, project server, outlook, etc... would be inheriting from a base tasks content type, then you can have plans feed from one or multiple task sources/pools (in plan settings for example)
O365 / Sharepoint would be a no brainer for me if "TASK" was the same across: Outlook, Sharepoint, Planner, OneNote, Flow.
Changing or creating a tasks in one should replicate through the rest. Without this integration, I have to consider each tool independently against other tools of their individual merit, which is often lacking for other reasons. If I had this integration, I'd default to O365 99% of the time.