There was a big discussion about Lady J being subpar and it turns out she`s not as clearly OP as Sonnia/Perdita but can stand her ground and people had good tournament scores with her.
Lady J is especially vulnerable to Deliver Message and Cursed Object as she has to be in CC or play area denial.
I think she can be a decent scheme runner killer. In that case I`d value Vendetta over Badge of Office, as Vendetta can easily get you a normal flip (either walk + walk + strike or walk + charge and with attack flip and MI 7 you can get a lot of damage in as you just have to hit for a neutral flip).
Her main drawback are quite weak upgrades:
Justice Unleashed is Outcast/Resser only basically,
Last Stand is board dependant, if you don`t have severe terrain its not worth it IMO as With Me! is not that good as Marshalls don`t really have a CC oriented model. It might be a cool idea to charge your own model and push all Marshalls in and wiff the attacks.
Implacable: Bulletproof +1 is ok vs a lot of things, but if you want to kill Lady Justice the +1 won`t help. Moving out of her engage would be cool but she has 7 MI, most people will either try to kill her or push out anyways.
Vendetta: Limited, 2SS, used it once and regretted not having Badge. If this upgrade was not limited then I would consider it (4 possible MI attacks on charge seem nice or double walk + 2 possible attacks) but it requires a mid-high (AFAIR) to hit which is probablty better used on her Riposte.
I`m generally a big fan of J (as avatar suggests) and I had mostly good results with her especially after she One Shot Francois Lacroix and Two-Shot Ophelia Lacroix