I don't know about the danger of disconnecting,
I'm still a happy user of this service, but I found some quirks:
+ My experience is with a daily appointment, where the time changed each day.
I go into my note and change the reminder date and time.
Looking at my calendar, I see the appointment with the new date and time.
+or- And the old appointment; its disappeared from the calendar.
If I really thought about it, this is the way it has to be.
+or- I was using the Evernote "classic" link in my calendar entries.
Now instead, there is a unique Cronofy url.
Clicking it gets me my Evernote entry in the browser.
I wouldn't expect this url to be live after disconnecting.
+ You can go into the calendar and make an entry recurring.
It seems to work ok - but it creates complications; I'd avoid doing this