For me it depends on the placement of a gateway. If we are talking about a cluster of hardware appliances and they are situated on the perimeter, case of an external firewall, then I would use separate interfaces for external and internal connections, I even would try to use different physical switches for external and internal zones. I'm kind of conservative in that sense too. In case of physically separated interfaces and switches we have less possibilities to do some totally bad configuration (on switches mostly), we have segmentation of devices by functions.
But as Vladimir mentioned it is a different case with VSX and cloud gateways. And for totally internal firewall I wouldn't have any issues with using one bond for all networks. I tend to agree with Vladimir on the topic of gathering all 1G interfaces into one bond, better to leave 2 interfaces for possible future changes in the network.
I had one case when I couldn't use bond interfaces for external connections. I configured two physical interfaces from an appliance into bond, I added vlan interfaces on it, and the additional trick was that there was only 1 external public IP address for this office. So I needed to configure private IP addresses (192.168.X.X) for cluster nodes and public IP address for the cluster IP. Usually this setup with different IPs work without any issues, just need to add a proper route on the cluster. But it was not working with bond and vlan interface, when all these things configured in the same time. It was R77.30 with the latest Jumbo Hotfix available at that time. We contacted Check Point Support, they took a look, we tried to fix it, but we didn't have too much time for this implementation and for all possible debugs, so our conclusion was that this setup with these three things combined is not working (bond + vlan + public and private IPs on one interface). When I used only one interface for external connections (+ vlan + public and private IPs on one interface) it started to work.
At the same time we had 3 interfaces combined into bond for internal networks, so it was not a configuration issue or some problem with switches.