I find the main weakness in a non PvP fire rush deck is the fact that your creatures die too easily, take that weakness away...
Quanta denial isn't really a weakness as bounce is a passive ability for creatures.
Nightmare is one weakness, a two+card combo to take down one card is op in my eyes.
Being able to target the creatures as normal isn't really a weakness...
Otyugh/scarab - take a while to set up and if you have excess quanta this is fine, if you don't have 7/8 cards in your hand this doesn't affect your draw at all
Each use of Ren will return a creature to the hand [+1 card advantage]
Each use of Hasten will add a card to the hand [+1 card advantage]
Both Hourglass and Fendhuang slow down a Fire Rush deck but provide card advantage that allows the deck to be sustained in the face of CC.
So that weakness is removed at the cost of being weaker against pure rush decks (CC + Rush > Pure Rush > Ren Rush > CC + Rush)
Quanta Denial prevents the card from being replayed.
Being able to target the creature is a weakness when you are comparing Ren to Immortality.
Otyugh/Scarab get endless growth against Fenghuang and force the user of Fenghuang to pay much more quanta 6
|5
just to return their creature to the field each turn.