But why is it unfair?

Some of the cards create card advantage, but there are a lot of cards that create card advantage without discarding but presumably you don't think those are unfair?

Discarding is just the hook, and all these cards have a high deckbuilding cost. There are plenty of cards that dont involve discard that say do x to create a y in hand, its not much different. Lost soul is a really bad card in any other deck, you need to build your deck around making the most of the synergy.

So with a highly synergistic package of cards, the best way to judge whether they're unfair is how the deck performs as a whole, and as of now I dont think the deck is overperforming. Its solid, but not oppressive, it has plenty of counters. Lost soul is a good card in the deck but this is the only deck it really fits in, so it only matters if the deck is too good.

Rummage was nerfed because it allowed discard decks (and ez-draven like you identified) to generate too much value too cheaply, and it was in a deck that was overperforming. Imo, if it hadn't been in deck that was overperforming, there would've been no point in nerfing it. Its okay for cards to be strong.