The plasma sword wielded by one of the orven martial lesser phase lords was not an inferior weapon.
Although it was not as unnaturally sharp and penetrating as Saint Piercer arms, plasma swords were powerful enough in their own way.
They were also incredibly expensive to construct and operate. They consumed energy like nothing else, but it was usually worth it because their damage output was just as exaggerated!
The one wielded by Zren-Divar was more powerful than most, not just because of its much greater size in comparison to the Scarlet Ember.
Another reason why it was so powerful in the hands of the orven phase lord was because it was able to tap directly into the large power reactor integrated inside Zren-Divar's raiment to fuel its extreme hunger for energy!