I've come across How can I show multiple Google calendars in iPhone's calendar app? but the topic seems to be dead already and doesn't solve my problem for now.
I'm using multiple Google calendars and successfully managed to add an Exchange account on my iPhone. The first problem was the exact email used for login to that account. Only @googlemail.com seems to work - not @gmail.com.
Another possibly important detail is that I use the newer 2-token-login and I had to generate an app-specific password for that Exchange config. But I've managed that too.
Now it comes to the point where I shall open up http://m.google.com/sync on my iPhone - but this states that my device is not supported. I've found out that the german version counterpart seems to be https://www.google.com/calendar/iphoneselect which loads and asks me to check the desired calendars.
But no matter how much of my 8 calendars I select - only the primary is synced to my iPhone. So does this thing actually work at the moment?