I would actually say in the most general conceptually sense that Lelith is pretty darn good, though despite her fluff she's actually better at dealing with mobs and would tend to want to avoid amped up solo characters.
Now, in a more specific and driven competitive build she's still not bad, it's just there are quite a few options that do as good or better than her, and do so for less point expenditure, which basically relegates her downward on the competitive scale.
Let's just consider the important parts;
Lelith vs.
MEQ - 11 attacks -> 7.3 hit -> 2.44 wounds
Lelith vs.
MEQ w. Furious Charge - 11 attacks -> 7.3 hit -> 3.66 wounds
So, she's going to kill off about 2.5-3.5 Marines on the charge.
Archon/Succubus w. Agoniser - 2 dead Marines
Baron Sathypants - 0.925 dead Marines
Asdrubael Vect - 4.15 dead Marines
The above then gives you some idea of how she's stacking up versus other
HQ choices. Now, only an idiot will actually try to claim Baron Sathypants is any good at all in assault versus
MEQ, but people do seem to think he's for some reason an assault guy, and I just like to be able to mock him

but we can see that for 73% more points you get about 150% damage increase via Lelith, even without
FC, and that's a good value.
Comparing her to our supposed ultimate beatstick Vect, we can see she actually stacks up pretty functionally in a point for point comparison. He's wracking up about a 100% kill increase in the best case comparison yet the cost increase to Vect is 156%. Now, he *is* better versus special characters, and that needs to be considered, but clearly he and Lelith are vaguely in an appropriate cost arc.
So when you look at the other named
ICs and want something that inflicts damage in h2h combat Lelith stacks up *very* nicely.
The thing that punishes Lelith is comparison to the cheaper
HQs. The Succubus, at a saving of almost 100 points(!) will kill almost as much as Lelith, and actually will be slightly a better option as a dualist versus enemy beatstick
ICs. Wooosh! Heck, the Archon can be cheaper than her and have a better overall survivability thanks to his Shadowfield.
So, Lelith has a special little nook, and has some nice little tricks via her hair and her ability to punish swarms, but she does have an awkwardly precarious position as far as her viability, and you have to understand what you're getting from her. She can be competitive, but as you build your list more and more focused on competitive play she starts to become a big question mark as to whether what she provides is worth the hefty point cost, since she does have a somewhat limited number of optimal assault targets.