The reason why the thresholds for the Divine Powers and Spells obtained from the Legacy Stone of the ancient Celestial Court are considered insurmountable is mainly because of examples like these:

For instance, cultivating the Nine Revolutions Immortal Body Technique requires such rare natural treasures that it's virtually impossible for anyone to reach the ninth revolution; even the eighth has never been achieved.

In the hundreds of billions of years of the Artifact Spirit of the Legacy Stone of the ancient Celestial Court, only two people have ever cultivated the Nine Revolutions Immortal Body Technique to the seventh revolution, comparable to the level of Supreme Quality Innate Ultimate Treasures.

Take the Yellow Spring spell, for example. Although it doesn't require auxiliary cultivation resources, it demands an enormous amount of mystic power to nurture the envisioned Yellow Spring, with a massive number of 530,000 Yellow Spring Rune Script imprints.