As an owner of a Fantom X since it came out years ago, i can tell you that the Fantom workstations pack serious power. 4 oscillator subtractive synthesis. The G came out after the X. It's likely to be loaded with even more synth waveforms. I know it has much more of everything than the X
Speaking about the X...
There's multiple filters in there as well as distortion (called boost) and ring modulation. The voice structure is not fixed, you have 10 different ways to organize osc/filters/ringmod or boost. You can definitely make your own patches. It's subtractive synthesis but with 1000s of waveforms to begin with instead of a few.
Tons of enveloppes, multiple lfos, a mod matrix, realtime timestretching and much more. If i remember, you could even use lfos or enveloppes to control its effect processor parameters. I use mine as a midi sequencer for my gear, effects processor for other machines and of course i use it as a synth. All at the same time. I did some timing tests and it held up very well. Sequencing a drum track while loading up the synth engine to the max polyphony, the drum track still ended up with a tight timing.
This is acid techno i made on just the Fantom using 909 samples for drums, the rest is mostly from built-in waveforms. No external gear was used except a behringer knob box to control more things at once. All acid sounds from built-in waveforms and filters :
quikphix records: phix 79
I love my Fantom.