MF is cursed with RIOTHATESMEITIS
It is a thing where they take their worst amateur designer to remake her. They obliterated her ult damage potential when they made it physical. Then, they took the pyhsical scaling off it and moved it to W. W now applies on ult for what appears to be 0 damage.
Ap scaling is still amazing but they remove ap scaling on W which was AMAZING. They put an ap ratio on Q for mage fortune but that ap ratio was already there with the old W because each shot got 20% your ap ratio AND STACKED.
Now in order to make your ult stronger, you need physical pen. But in order to increase the ult scaling, you need ap. In order to increase your W scaling, you need AD, but in order for your W to do any damage at all, you need magic pen...
She literally has no optimal build path. I just build her 5 ap items and last whisper and go all in with my ult.