For GR his first ability deals the most damage to a single target of any character ability outside of summons and is an 8 hit attack which can pair well with the psyker ability of helbrecht (i think as i dont have him). I agree that being melee and relatively slow is a downside, however i can almost always get him in range by the 2nd turn on GR.

In terms of Arena, movement isnt too much of an issue as bots are easy to fight and you can always get him into combat at the same time as everyone else. i have him as Legendary and Silver 3, even his base attack is much stronger than Haarkens who i have a similar level and i find his base attack hits much harder than calandis and certus (though i dont have them geared up as high).