Is WoL the only one able to master/handle all the classes at once?

[Dragonexx]

What is the canon explanation for that? Does the sheer power of the gear overwhelm the user and melt their face like they just looked into the Ark of the Covenant?

Possibly. At least, the lore for one of the monk Artifact gears describes it as doing that.

“Only elders of the Fist of Rhalgr are permitted to don this attire. Though the thread used in its creation is the same as that woven into the standard temple habit, the veteran monks looming the tantra cloth imbue each stitch with concentrated energy, resulting in a significantly higher rate of aetherial conductivity. A monk’s chakras are thus greatly expanded when swathed in such material, but without the training required to moderate the excessive outpouring of aether, an unseasoned wearer would bleed out his entire life essence in a matter of moments.”