Under normal circumstances, Dyon could make use of even Spiritual level weapons without much of a drain. However, this wasn't because transcendent weapons were easy to use. More accurately, it was because beings below the celestial stage couldn't use Spiritual level weapons to their greatest degree.
There was no question that using a Spiritual level weapon was better than using a grandmaster or lesser weapon. However, what was important to note was that this was mostly because of the quality of the weapon as opposed to anything else. For example, pouring a set amount of energy into a master weapon will always have better effects than pouring that same amount of energy into a common level weapon.
This was why using Spiritual level weapons wasn't taxing – because you could always choose how much energy you used…. Usually.