Hi,
I'm also with Zen - I have a FB 3490, hoping to get the 7530 when my contract is up for renewal. I'm on an ECI cabinet and find the FB syncs at a lower rate than other modems I have owned that don't match the cabinet CS.
That said, I find the Fritzbox to be very stable - I have it set in bridge mode and use pfSense for firewall / router duties.