I totally agree with waht was said.
I commonly use Outlook (the real and complete one from Office) as a mail client, which I found has more useful features (at least for me) than the Windows 10 Mail app.
On the other hand, I find the calendar display and management features of Outlook particularly cluttered and ugly and so I use the Windows 10 Calendar app to manage my multiple personal and professional calendars.
What I hate with this new Outlook, is that the UX/UI is a dramatic regression compared to Calendar as it is cluttered with unnecessary information at the point to be nearly illegible PLUS you can't prevent it to download your mails and thus have double notifications.
I will continue to use the old app as long as possible, but I'm already looking for alternatives matching my personal needs.