One of the main disadvantages of doing that is losing most of your network management options. For instance, I like that I’m immediately notified if a new device joins my network. If I remember correctly, you lose that, device blocking, network usage by device, and HomeKit support, and a few other things if you put your eero in bridged mode. Better to bridge the modem if possible which it sounds like Johnny was able to do.It might be easier to set the Eero in Bridge mode instead of fighting with the cable modem/router.
In the Eero app, Go to Settings -> Network Settings and then change DHCP & NAT to Bridge.