MOD-7 has the equivalent of 6-op architecture (a la DX7) but with the ability to custom-configure algorithms that didn't exist for the DX7, and with a wider choice of waveforms for each oscillator. You have the additional resources of a noise generator as well as PCM as modulators, so it would be fair to say that it's 8-op, and modular as well.
When doing MOD-7 programming for Korg I found that PCM, used as a modulator, is best for creating "industrial" or "ambient noise" type sounds. Noise, on the other hand, is a very useful modulator. And unlike the DX7 architecture, where if you wanted to use noise as a modulator you had to raise its feedback up to 7, which kind of wasted an op. Here you have a dedicated noise gen, so that spares you an op in those circumstances.
In terms of the sound, it can produce everything you'd expect from FM and then some, including some very WARM sounding patches. I think it's a great model, and IMO anyone who's into FM sounds will probably find the model to be endlessly versatile, even surprising in terms of the sounds that can be created.
In terms of comparing it to FM-8, there are sonic differences. Whether you like one over the other is purely a matter of taste. I find FM-8 sounds to cut a little too much in a track, whereas MOD-7 sounds a bit more "natural". These are, of course, subjective terms. But all subjectivity aside, you can't get MOD-7 as a plugin for your computer-based DAW, so if you want MOD-7 you... have to buy a Kronos!