Shadow damage weapons
How would you rate weapons with shadow damage bonuses? Are they a bit ill-suited for the current end game monsters? How do you compare them to the other elemental swords?

Not great for Vanaduke. Why? Zombies.
I posit that the cursing effect and long range of the faust/gran faust charge attack make it worthy at any point of the game. At the very worst it's as strong as a troika of the same star value (or better). There are better choices (avenger/divine avenger for example) against undead, but this is about shadow weapons.
Just like elemental damage, shadow damage has enemies that are weak and strong to it. Luckily gremlins are weak to shadow, and since they are perhaps one of the most annoying late game enemies, I'd say shadow has an advantage.
Faust/Gran Faust are fantastic for everything but the things that resist their damage.