i have a 3900x very good cpu but i think there a lot room for improvement in mixdown . i think for now the use only 3 or 4 core from what i tested myself.
for a lot of other product i use i love my 3900x ! i use melodyne editor, RX izotope ...and i work with video sometime... 12 core is good !
for a more technical point, i think 9900k is the last of intel in this generation, the x370/390 platform is nearly dead now, so i choose a amd x570 (with thunderbolt 3) . for intel i recommend wait te 10th core generation to see if they want to give more than 8 core to mainstream ... and see a price change to compete with what amd offer now. for me amd is for now a better solution , and more cheap !