Besides the fact that guns are often better than swords (except against Gremlins), there's imbalance between different types of swords. Specifically, the Brandish and Spur, a pair of 2-star swords, have a better damage output than the Khorovod and Vile Striker, both 4-star swords. The Vile Striker in particular is nearly useless, since its lack of knockback means you can never finish its combo, and it doesn't even poison that often.
Who even knows how much better the Faust and Avenger (or even the Sealed Sword) would be if anyone could actually get one.
I'm not sure exactly what you could do for the Vile Striker, but either the Khorovod and Troika need to do more damage, or the Brandish and Spur need to do less.
I know, and the answer is a lot.
The Avenger is like the Spur and the Brandish put together, but with extra damage.
Faust has a chance to inflict curse on you though, so wearing Skelly armour might be a good idea.