If you want a guarantee, buy a toaster.

The title of this piece is a quote from Clint Eastwood. While Clint may not be an eminent philosopher, what he says makes a lot of sense about the world if one thinks about it. "Guarantee" is a word that is not as widely used as it once was. Back in the day, products and services were guaranteed. Clothes would fit, shoes would hold up under heavy use, cars would run reliably, planes would arrive on time. Now, our vocabulary has evolved to use the term "warranty" which seems much less definite and more time-limited. Many items that you might buy don't even have a warranty.
One of my favorite businesses, which still uses the term Guarantee, is LL Bean. The founder, Leon Leonwood Bean, said to treat people as human beings, give them good products at a reasonable price, and they'll be back. The only fault I can find with the company is that, sometimes, an item is out of stock and back ordered. That being said, it you buy something from them, it is guaranteed to work or fit as expected. If not, they will fix it, replace it, or refund your money without question or hassle.
While businesses and organizations my strive to "do the right thing", there are varying degrees of success. The downfall for many is that, while you may have the best of intentions, you have to rely upon other people to help get the job done, and they may not share in your work ethic. Put yourself in a situation in which you are, for example, trying to get a project done but, in order to do so, you need information from someone else. No matter how motivated you might be, if the other person does not get what is needed to you in a timely manner, you don't succeed. If I am solely responsible for the outcome, I do whatever is needed to get the job done, hopefully correctly, and on time. When relying upon others, my desire is till there, but I am reluctant to promise the same results.
I was recently asked by an individual if I would guarantee something. I responded that, even though I would like to, I could not because I had to rely upon others for the end result. It's like making a promise on behalf of someone else; it's just not a good idea.
So, we live in a world that has evolved to limit our expectations, at least I most cases. People upon whom you rely to do something may call in sick, items you need may be delayed in delivery, plans may be made by people who don't have to actually do the work to make the project successful. Today, giving something your best effort is, in a lot of cases, the best that can be expected. Don't ask Clint, or me, for a guarantee.