We know almost nothing about Unohana's strength. Her battle data place her 3rd among all captains, just below Aizen and Yamamoto, but she has never fought, so we don't know what her power is and if she has any limits to it. The actual sentence about her in Bleach bootleg reads "She rarely has an opportunity to serve on the front line, but her natural offensive abilities are at their peak! Could it be that the most terrifying captain of all is..." That's pretty much all we know about her offensive capabilities. Make of it what you will.