He is OP. By himself he is one of the most powerful and versatile toons in the game, no question. Well modded and geared he can beat 2-3 other toons in arena, meaning you never count him out.
What makes him the OP meta is the relatively recent flood of rebel and light side love. Wiggs, Chaze, R2, GK, Fulcrum Ashoka, and even classic rebels such as raid Han, Leia, and Old Ben getting some love and/or synergy have really pushed cls (and rebels I general) over the edge. On average, light side characters are much faster than dark side ones, have greater tenacity, better unique abilities, and virtually limitless synergy combinations due to the wide variety of competitive light side toons.
Its why there are almost no dark side teams in the arena now. When the average light side toon can be faster than a dark side toon under a Maul lead, there isn't much chance for dark side to compete. Plus, there is almost no synergy among dark side toons without going faction-specific, and there just isn't enough variety, farmability, or value in doing so.
We need some serious dark side love. For example, give emperor palpatine a unique that says his attacks don't trigger unique abilities from rebels. Give dearth Vader a huge damage and defense boost, and a new ability "force choke" (because, Vader, duh), which removes all protection from the target and gives stagger for two turns. If the target is a rebel, give permanent buff immunity (cannot be dispelled). Make his leader ability include "rebel enemies cannot gain additional turn meter as long as Vader is alive". These changes would slow down the ridiculous rebel assaults, and allow f2p players more flexibility in arena.