BM had the strongest df amongst the top-tiers until Nika fruit was revealed. Just like ope ope, it felt like Oda was deliberately nerfing her df usage in some situations. For example, she only healed her arm instead of a full regen (or even her broken ribs when she mentioned it), another one is why did she only use 1 year of her lifespan? Right now, we can chalk it off to that being limits of her fruit and that sucks ass.
Misery never hit anybody in the manga, it would have been so much better if her final move was launched like a shockwave before it turned sentient. She was capable of doing much more destructive aoe back in WCI. Oda really rushed the entire fight to not overshadow Luffy’s fight, such a shame…
Anyways, her df still remains to be one of Oda’s most creative designs with crazy potential. It’s in the same tier (personal ranking) as the vague dfs like ope-ope, paw-paw, and nika fruit.