This is the accepted Sefardic practice and this is also the ruling of the Vilna Gaon. The Rama, however, cites the opinion of Rabbenu Tam, that a vertical mezuzah is invalid. Taking this opinion into consideration, he states that the best way is to put the mezuzah on a slant. Since the mezuzah is neither vertical nor horizontal, it is valid according to both opinions. No matter how reliable the Sofer is and no matter how costly the Mezuzah, even the best scrolls can develop problems over time which are detectable only by highly trained and certified professionals.
Mezuzahs tend to crack with age.
Additionally, moisture can cause the parchment to rot, causing the ink to stick to the back of the Mezuzah and thus erasing the writing. These are only a few examples of the more obvious problems the examiner will look for. Scribes who are certified are trained to pinpoint otherwise undetectable problems. Even the tiniest flaw in a letter, which can develop over time, can render the Mezuzah possul. In addition to the careful scrutiny of the Sofer, each scroll is computer scanned to verify textual accuracy and double-check for overlooked errors.