I feel like this artifact system or (RNG) does not respect or care about the time players (who loves this game) puts into farming Dungeons. I did over 50 runs in fire knight stage 20 and only got about 5 artifacts that are worth keeping the rest are a bunch of flat stats and mix. Here's my idea on making this system better. 

Rare artifatcs: should guarantee 1 (%) substat

Epic artifacts: should guarantee 2  (%) substats

Legendary artifacts: should guarantee 3 (%) substats 

Plarium, this will also help your artifacts packs sell out more because the players now have hope of getting a better chance on good artifact pieces. Therefore a very good reason to buy the packs. 

RNG is good but too much will only lead to frustration and player pupulation reduction. But I suppose you don't care about that. I strongly hope you take this idea into consideration.