It’s not uncommon to hear of someone who promises a certain item to someone on their death, or mentions the way their will is set up. What is less is to find out that person actually followed through with that plan – in writing.
In estates law, as in almost everything, what is written down tends to take priority. While a court will sometimes hear about a promise to leave an asset to someone, and will more rarely order it given to that person (depending on some very specific circumstances), the more likely situation is that the person promised the item simply won’t get it. Everyone has the right to change their mind; telling your friend that you were planning to leave something to your niece certainly doesn’t hold you to it.