I love this and it's not really as dangerous as it might sound.
If the project goes badly, you are going to have a bad time regardless of if you offered a guarantee, and you might end up just refunding them anyway if it really went that badly.
Being upfront about offering this reduces their risk and in the vast majority of cases people will never ask for a refund.
I like your way of thinking about it and how you worded it.
Regardless of whether a guarantee is offered upfront, there is going to be a mess if the client isn't satisfied. The way I think about it is: I might as well use it to everyone's advantage and be clear about it upfront.
And there's a substantive difference between all parties knowing things could go wrong versus one boldly stating upfront that one will try to do the right thing. This will close more deals and offers additional value upfront to the client.
Of course, it helps if one addresses other issues like filtering out likely to be problematic prospective clients, but that's another post / series of posts for another day. ;-)
If the project goes badly, you are going to have a bad time regardless of if you offered a guarantee, and you might end up just refunding them anyway if it really went that badly.
Being upfront about offering this reduces their risk and in the vast majority of cases people will never ask for a refund.