are the best models with the worst rules in the DE codex, the main culprit being this assault unit cannot assault units in cover, so here's my shot at a fix that also adds a cool flavor
Special Rule: "Shadow Brand"
Mandrakes ignore the effects of cover in assault and shooting
Flavor: when an enemy seeks cover he is also enveloped in shadow, the hunting grounds of the mandrake!
And an addition to Baleblast:
"whenever a unit with this rule has at least one pain token each model may make close combat attacks at ap4"
The mandrakes are now capable of assaulting any unit, and when a pain token is gained they get that "ignores cover" shooting attack that's so fashionable in 6e.
As for Scourges, I feel that they just don't really stand out next to Reavers
To represent their extensive experience in battling over the bloody skies and lightning cracked spires of Commoragh, Scourges get...
While Reavers rely on blazing speed and Hellions have numbers, the Scourges are the sharp eyed killers who gun down their foes at extreme range. You can already imagine the bloody dynamics of Commoragh Spire War with their distinctions!
Scourges now stand out unique among the DE as a highly mobile interceptor that can also foil enemy air mobility.