Some text extract from SAFWY, I think this should confirms Mugetsu is not just a vertical cut, but as shown in the anime it crumbles away and disintegrate the whole body, and the difference of power between Aizen and Ichigo might even be greater, while Cien and Roka where somehow comparable, then Roka got this immense boost, Ichigo was already stronger than Aizen and even boosted himself way more with Final Getsuga Tenshō.
また、あの甘美な殺し合いの——「死匂」に塗れた舞踏を……。
どこかに向かって手を伸ばした所で、彼はその指先が崩壊していくのを感じとった。
皮膚の表面が次々と霊子が零れ落ち、舌から赤い肉の色が覗き始める。
——待て、待ってくれ。僕は、必ずまた殺し合うと誓ったんだ。
As he reached his hand out toward somewhere,
he felt the fingertips beginning to disintegrate.
The surface of his skin steadily shed reishi, and from his tongue, the color of red flesh began to peek through. Wait, wait for me. I swore I would definitely fight to the death with you again.
「……僕は、僕はまだ、死ねないは……駄々を捏ねる子供のような表情で、声にならない叫びをあげるシエンだが——そんな言葉に惑わされる事なく、暗闇は滅びの歌を歌い続け——シエンを包みこむ「死」は、ゆっくりと彼の身体を崩壊させていった。暗い場所から生まれた狂気を、再び暗い場所へと還す為に。ロカは、かろうじて生きていた。あの力を発動させた後、具体的に何が起きたのかは解らない。ただ、周囲にはもう何も残っていない、という事だけは理解できた。散在していた斬魄の瓦礫はもはや、一つも残っておらず、辺りには、砂が流れる音だけが響き渡っている。
“…I still can’t die yet…”
With a face like a child throwing a tantrum, Cien let out a voiceless scream, but the darkness paid no heed to such words and continued to sing its song of destruction.
The “death” that enveloped Cien progressively broke down his body, in order to return the madness born from darkness back to darkness once more.
Roka was barely alive.
She didn’t know exactly what had happened after unleashing that power.
All she could understand was that nothing was left in the surroundings anymore.
The scattered rubble of the Zanpakutō was now completely gone,
and throughout the area, only the sound of flowing sand echoed.