That she can't be stunned is pretty useful in dungeons and most everywhere. That alone would make her worth developing. Also, as a flyer, she is immune to hero bombs, which can be a big deal in dungeons too. She can clear a path directly, like all flying heroes, instead of wasting her time running around walls and maybe getting killed in the process. And that in turn keeps her proc going. Which also helps prevent *other* nearby heroes from running around walls to kill a nearby target, since the target will vanish as HB attacks it, letting them reroute to more sensible targets. It's a nice indirect bonus of flyers.
The squishy thing is true. My best one has scatter, and she did do well in HBM 3, though., through AF so far. It's really helpful to have a healer in her group, to tide her health over until she procs, when she can start healing herself. Also, it helps a lot to devo her and max her skill, since both directly affect her health and, well, she's squishy!
Nobody replaces gunslinger at this point, though.
However, Heartbreaker can deal a lot of damage, and so is a worthwhile candidate for Lava 3 or 4.
If you want to develop her for damage, give her lots of crit traits and a damage augment like victory lunge or whatever. If you want to develop her for debuffing, go for accuracy, as she can't debuff anything she can't hit and top players are filling their roster with dodge heroes. Plus all the crit in the world doesn't matter if you're constantly missing.
If you want a general purpose hero, a combination of dodge and accuracy will work nicely, but so will a random assortment of almost anything, from damage to hitpoints. She's versatile.
Fit to suit. For instance, I don't have a wargod or bulwark heartbreaker, I have scatter, so I didn't choose to emphasize damage traits. I wanted a good debuffer, so she's scatter with accuracy traits and a couple of dodges.