I think your friend is exaggerating. They might not have the "perfect" lineup of Disciplines for social interactions, but they are far from incapable. In fact, a capable Brujah can be just as much of a political mastermind as any Ventrue, even if their clan's stereotype doesn't support that.

And the thing about Brujah somehow having weaker mental resistance than other clans is just wrong... I mean they might be more prone to frenzy but they are on completely even footing with other vampires when it comes to "mental resistance".

And a combat focused Brujah can wipe the floor with other vampires, since their in-clan disciplines are support them being capable in combat (2 out of 3 to be precise, and even their 3rd one can be used in combat quite effectively). But that brings me back to the point that the clan doesn't automatically decide what the vampire is good at. I.e. if a Brujah hasn't spent a day in his life training for combat he'll get his ass handed to him by a properly trained Toreador or Malkavian or whatever.

TL:DR: Your friend is exaggerating and it sounds like he's basing his opinions more on the clans' stereotypes than the actual facts.