None.
Just the issues that will come from having to double configure port forwarding, static routes etc.
Setting it up in bridge mode is extremely easy. There's a step by step tutorial kitz has made for the vmg8324 but it's the same procedure on most zyxel devices.
The benefit of bridge mode are that the modem only handles the xDSL connection, nothing else. Configure, connect, leave forever.
You can then make any changes you need to your router any time you want without worrying about breaking the dsl sync. DHCP, NAT, PPP, firewall, static routes, wifi, all done the the R7000.
If you need help setting up bridge mode just ask.