The downside is this old HP LSI Concorde modem I was going to use. I installed it in a spare PC and... there are no drivers auto-installed. Did some digging and apparently softmodem cards had the bare basic hardware to work and relied on their proprietary drivers. Each manufacturer did it differently so Linux driver support is spotty at best.
I did find this martian-modem driver package, but the installer doesn't work on modern kernels.