My experience: I bought Firehawk FX and liked it very much. Never had a single issue and worked on some preset which gave me great tones.
My setup is simple: FHFX into Harley Benton GPA-400 solid state 400 watts power amp, into 2x12 V30 guitar cab, switch on line out, cab simulation on, and to my ears, in to my band's buddies ears, sound great.
So out of curiosity I bought PD 500hdx out of a great deal, and I started a comparison one to one.
To me FHFX sounds better.
I exact same settings with amp Soldano SLO clean channel + tube screamer and the two units sounded very very different, to me FHFX much better.
I did the same for all my presets and the results was the same: the two units sounded very very different.