I don't have a mesh network setup yet, as I'm figuring out how to cleaning get a CAT6 line outside and hidden through walls for a GR60, but are you going directly from the FIOS router/modem into the GR10? Or does it run into a GX20 and then into the AP? If you spelled that out initially, sorry! I read over twice to make sure I didn't miss.
I have FIOS, too, and run from the modem/router into a GX20, then to a GS110-8P and then to the GR10 with zero issues and three SSID's (all with different settings for restrictions and bandwidth). Moved from Comcast and my speeds have been dramatically better. I produce Virtual Events/webcasts for a living and test UC technologies so run TONS of upload/download traffic from an obnoxious amount of devices to produce content and test tech.
Is there any theoretical concerns that could be causing security issues by not going through a MGo security gateway first? I can't imagine Verizon's equipment creating any kind of firewall issue with the AP.
The most annoying issues I've had is that all the FIOS set top boxes run off FIOS's SSID by default and sit outside of my LAN so device controller apps don't work unless I temporarily switch to that SSID.