So they are in the Clash of Kings - Feast for Crows period, roughly the same courtiers but Beric is leading them in COK and Lady Stoneheart in FFC. The former has good diplomacy and decent traits, whilst Lady Stoneheart has a rough go traits wise, basically 0 in all stats.
The benefit of the Brotherhood is they have special events they grant them event troops often when successfully besieging holdings, as they are at war and their starting event troops are raiding locked, but you can always hire mercs to take terrority. They also will get a set amount of gold from these events as well.
So whilst you start with a small number of troops (3k), that number can grow quite a lot assuming you don't get wiped or fight major battles.
The best aspect of this unlanded Duchy title is even after your current wars end, the events will always fire as long as you have the title. Whilst your current wars are tricky to get out of, eventually you'll be able to either white peace or someone wins the throne you're not at war with (and you get re-vassalised). If you win or lose your starting war, you WILL lose your title, so you don't want to do that. As Beric you get your old county back, as Catelyn you lose instantly, so, yeah.
But if you white peace, you keep the Brotherhood Without Banners Duchy title, with the event that keeps firing and if you claim a landed duchy, you can change succession.
In FOK you have a much better chance of surviving, as Robb's main army isn't next door and there a slightly more Rhllor characters to try and recruit to be your heir and eventually when you become a dynasty including...of course...
Victarion, outside of Stannis the best Rhllor character to play as, that reasonably can be recruited to your court and assist you in surviving into greener pastures.
Any other elements you can think of that will test this out? I'm thinking of being anti-slavers on a never ending conquest to rid out the world of the foul practice, possibly be settling in Lorath.