The B-wing is probably fairly costed, it's simply in a bad place right now due to the under-costing of Jumpmasters. Assuming four hits from two JM5K plasma torpedoes, the only possible way to survive is to roll evades on both of your dice, and hope there's no Direct Hit crit. Even if the Jumps miss with one shot (against the odds, but possible), blanking out on both dice is still very likely. Even without that you're basically left on one hit point, easy fodder for a turret with one of the best dials in the game.
The Rebels in general though do have an issue at the moment.
- Scum have massive offence in the form of Jumpmasters, with reasonable defence and good maneuverability.
- Imp aces have massive defence and maneuverability, with OK offence.
- The Rebels... well, most of their ships are low- to mid-range defensively, but this is inadequate against torpedoes. They have a severe maneuver deficit (A-wings and super-Dash excepted). And they're merely OK on offence - which is inadequate against Aces, and doesn't do damage fast enough against Jumpmasters.
At the moment there are definitely rebel builds that can take on aces. And there are definitely rebel builds that can take on Jumpmasters. But there are very few options if you want to be able to take on both of them, and those options just aren't as good overall.