But isn't this just a fractional reserves in this case?

How will you have the reserves to pay out everyone who wishes to cash out all of their Unum 1 month later if the price of ETH as dropped by 50%? Where will you have the funds to do so?

Edit: I see from your website you have a penalty in place for when your reserves drop below Unums in circulation - clever play! (https://unum.one/#!/learn#reserve-deficit-sell-penalty)