I think that Unohana's Bankai has the ability to melt the flesh off of her opponents (provided I take the panels at face value). Why do I say this? Let's look at the Japanese characters behind Unohana's Zanpakutō, Minazuki: 肉雫唼. I'm no expert on Japanese, but the Bleach Wiki (which I pulled the characters from) says that the literal translation of those characters is "Purify the Flesh." Unohana was recognized as Soul Society's best healer prior to the Thousand-Year Blood War arc, so the name of her Zanpakutō suits her. Now, we delve further into Unohana's personality, and discover that she is actually a blood-thirsty serial killer, much like Zaraki was when we were first introduced to him. What's a more fitting Bankai for a serial killer than one that literally melts her opponent? Okay, I'll admit that my explanation sounds a bit hasty right now, but let's look into her Shikai. Minazuki's Shikai form's ability is to ingest Unohana's patients and use its stomach acids as potent healing medicines. However, what if that ability may have been a modification of Minazuki's standard abilities thanks to Unohana's desire to hide her ever-violent nature?
Just my 2 cents on all this.