Because spike nades do damage over time. You can realize it is doing damage and get out of it. A lightning grenade does damage on impact and makes the opponent one shot. The two really are not comparable besides the fact that they have big AoE's. And I have noticed more and more recently that lightning grenades are damaging me even if they hit the ground next to me facing up. They have such a big AoE that they are forgiving for small mistakes in placement. And just like tripmines they can hurt the user. That doesn't determine if they are OP or not.