My Mids' says this takes ToHit buff sets and nothing else. I can put regular damage, accuracy, end rdx, and recharge enhancements of all types in. Only the set selection appears to be bugged. It's this way for both Arachnos Soldiers and Arachnos Widows.
Has already been discussed here (http://cohtitan.com/forum/index.php/topic,2673.0.html).