If you put your modem outside of your firewall (which is where mine is, in spite of the fact that I haven't successfully connected yet), your exposure is no worse than being attacked from another host connected to the HamWAN network*. You *do* have a firewall on your network, right? ;^)
* Well, ok, speaking as a professional security geek (which is what I do for a living), it *is* in fact very slightly worse. Assuming the firmware of the modem could be compromised to launch attacks, it's a higher-bandwidth lower-latency connection to pound on your network from, which, in theory, is less secure. But given the speed of the HamWAN network, the delta is pretty small, and given that the modems run a semi-proprietary (and fairly uncommon) OS, the odds of the modem itself becoming a leapfrog platform for staging attacks are pretty insignificant. And again, assuming you've got a halfway decent firewall in the middle (ie, not just a cheap consumer device that does NAT, but an actual firewall), I wouldn't worry about it.
Jeff N0GQ