2 Corinthians 1:20 (NKJV)
One of the worst things that a person or a company can do is to advertise or promise more than they can actually deliver. If someone told you that they could help repair your computer, but then did not even know the basics of how a computer operates, it would be bad. If a company advertised that a product could do a certain thing, but the product could not do half of what they said, they had then over-promised something. In either case, the person or the company were shown to be untrustworthy.
It is a sad but true fact that sometimes people and companies promise things that they cannot deliver, but it is a wonderful and true fact that this is not the case with God! What God has said and promised to us in His Word is something we can count on. If there is something God has shown us in His Word, we can be assured that it is a statement of truth and we can act in confidence on what God has said. The question is not if God will be true to His Word, the question is if we really believe it.
Too often people and companies over-promise on something, and we sometimes attribute these same qualities in people and companies to God. We can easily become conditioned to think that there are no guaranties, and things are never quite what they are advertised as being. But God's Word is not "Yes, no, or maybe", God's promises are Yes and Amen in Jesus! God's promises are as advertised, everything God has promised in His Word is something He will do!