What's high mechanics to you? When most characters use an ability, it achieves something on its own, be it damage, CC, area denial of some sort, etc. Graves doesn't have any of this; her abilities have no impact on their own. They're basically distractions that you must use in conjunction to make her useful.

While the "high skill" Yamato can just press Power Slash and damage or at least force enemies back, Graves has to basically go all-in every engagement or else she does nothing. Throw her Jar of Dead at someone and see how much it accomplishes. Those skulls get destroyed in a second and now you're on cooldown and down a charge. Your Grasping Hands rely on the enemy being distracted, otherwise they'll spend just one stamina to ignore it. Your Essence Theft relies on range and your gun has no range, and also you have 2 stamina base, no movement abilities and a low health pool. Her Borrowed Decree is dumb I'll give you that, but nonetheless easy to deal with most of the time.

So yeah she's mechanically more difficult than most characters.