I think the power of ghostmare mostly comes from the power of the ghosts.
When your opponent has his two first ghosts out (which will happen rather fast with most ghostmares taking just a darkness mark and many time pilars) he is doing 14 damage per turn. If by this stage you don't already have a shield or some creature control in your hand, you are very likely to be dead before the end of the nightmare chain.
If the only counter you have are individual creature control spells, or creatures to outdammage the ghosts, your only option is to empty your hand by using them, making nightmare more efficient, and eventually ending discarding ghosts.
In most games ghostmare win with the opponent not even discarding one ghost. Just because the ghosts are powerful enough to kill someone who can't draw a counter to their attack power.
A fact is the ghosts with their 7/4 are the best midrange attackers in the game, ideal for nightmares even if you remove their "do damage on discard" ability, as they are both in the most powerful, in the most resilient (not killed by one rain of fire) and in the hardest to play for the opponent with their 6
cost (and in addition there is the synergy between nightmare and reverse time) ; a nightmare deck can probably work decently with some others like toadfishes, gargoyles, mummies, etc... But they both are less powerful and require less quantum to play, increasing the risk to see the opponent finally use them, and in elements not offering any synergy with nightmare.