The Dark Talon is garbage, full stop. Bad main gun, bad bomb, in the end it's a flying Hurricane Bolter. With no hyperbole intended, it's easily one of the worst units in
40k. On to the Nephilim, it's a little overpriced, but mostly the problem is that its weapons load-out is poorly planned. One Lascannon, even twin-linked, isn't going to reliably break open vehicles. The Heavy Bolter is a pea-shooter, and both the Blacksword Missiles and the Avenger Bolter are saddled with AP4, which render them all but useless against all of the
MEQ armies out there.
40k tends to reward units that are highly specialized, and the armaments of the Nephilim are either too scattered (a tank busting main gun backed up by middling anti-infantry fire) or too ineffective (a lot of middling anti-infantry fire) to justify its use.
That said, I run two Nephilims just because I bought the models, and want to get my money's worth out of them.