Yes; it's all RNG reliant but as reported lately in this topic something weird is happening with single pulls/jewels
Also IMO, 100% Eidolons > Core SSR > Niche SSR > Trash Tier SSR; it even goes for Whales; now, have you actually been lucky with how much you've spent? You might personally think it was a fair return but someone else with more Core Himes and or 100% Eidolons that has spend less than you might think that you got bent over and rammed by the RNG.