I was afraid of such situations but as I remember it never happened.
For series of reasons, probably... Opponents always give all their strength to destroy CO and, rather sooner than later, they succeed. If the opponent can't kill Cursed Obelisks, it means he/she is probably losing and you can play Anubians alongside with zombies and you don't need to spend Land of the Dead in that case. Land of the Dead can be the wave that follows if / when Cursed Obelisks is destroyed.
But of course, inconveniences can happen - even the best card not only helps to win but also deprives you of opportunities. I consider COs are fine here.