[Boron]

Some Devil Armies are also good . If you generate Devils from Soul Contracts they are anyways extremly cheap but also by normal summoning via lvl3/lvl9 blood spell you get Devils quite cheap . They are immune to Firemagic , so no Flames . And they are extremly good and tough troops also , so the Ghost Riders can't do much harm to them .

Another good counter would be mechanical men armies with good mages like Tartarians or a Wrather .
50 Mech Men + Wrathful Skies can most likely survive even 10 Ghost Rider castings at once .

Getting 10+ Ghost Rider casters takes a while also . Most nations have to use Demilichs for that , so you need probably enchantment 8 + Conjuration 9 for mass Ghost Riders . That probably takes until turn 40-50 and by that time you will have various counters vs. Ghost Riders already . If you are realy nasty you can charm/enslave mind the Ghost Rider commanders . Especially a Nation like Ryleh could do that rather cheap , just Starspawns with rune smasher + spell focus casting enslave mind . They have good chances to succeed then .

The only thing at which Ghost Riders is really good is taking out SCs .

In a current game my enemy did 3 Ghost Riders on my ID with 40 Fiends . He killed 11 Fiends before all Ghost Riders were destroyed .
11 Fiends cost 40 Blood , a loss of 40 Blood vs. 15 Deathgems on my enemies side , i think that's fair .

Also note that Zen did only change the requirements for Ghost Riders from Death 4 to Death 6 but he changed nothing with the costs . So Zen doesn't think also that Ghost Riders are that powerful . Banelords on the other hand got a cost increase of 50% e.g.