Without breaking the NDA, here's what I can tell you from my experience in taking and passing the ECMS exam late last year.
The self study guide is good, but the Learning Hub classes were definitely a better resource. Also, the book "Cisco Meraki Fundamentals: Cloud-managed operations" from Cisco Press was like a bible for my studying.
They can only ask you a limited number of questions over a limited amount of time, so they'll have to focus less on specific models and more on the product lines as a whole (focus your study time on big picture concepts rather than the little details).
All Meraki products/product lines are fair game, so I would balance my studying equally between the different product lines. Look at the exam topics - notice what is there, and what isn't there https://learningnetwork.cisco.com/s/ecms-exam-topics
You will have topics that you know better than others. That's OK; I personally had very limited experience with Meraki's MDM, but my experience with JAMF as an MDM was able to help me understand the overall concepts and then it was just learning the Meraki specific differences.
You need to know Dashboard cold for all of the listed products - Practice on dcloud.
It's definitely a passable (although not easy) certification, and with some good studying, you'll do great!
