You're right, the point is as long as you're below UV, then you can think in terms of dielectric heating is the important take away, because it's only when ioni

You're right, the point is as long as you're below UV, then you can think in terms of dielectric heating is the important take away, because it's only when ionization can occur that you start to care about cancer I guess. But no, the reasons why microwave ovens work have less to do with their frequency than UV interactions do, which are QM in nature. The only real effect wavelength has in classical EM matter interactions is penetration depth. Anything longer wavelength than visible light has less energy per photon and thus can't ionize, that goes beyond saying, may be I should have opened with that. The only thing that matters after that is intensity and penetration depth.