I don't think there's any.

In Bitcoin, 51% means owning the consensus of the network. Nostr needs no consensus. A note only needs to be stored by one relay to be delivered. Relays don't have to "agree" that you were the one who wrote that note because we don't have a "double spent problem".

For me the toughest challenge for the network is to be adopted by normies. We somehow need nostr to be more convenient and better-looking than centralized platforms with tons of cash.