Yes, Scorps and Spiders are much more used than undead units. Maybe the devs need to nerf scorps and spiders so that we will use undead units more.
Just kidding! If you lose 24 Silver Acolytes in battle on enemy territory and you lose 7 Scorpions, 70% of the scorpions survive, while all of the Acolytes die. If you add up the mithril costs, you lose a considerably larger amount of mithril if a squad of 24 Acolytes die rather than 7 Scorpions. I think it's 37% more. However; if you fight on your own territory, then the difference in cost-efficiency increases even more! 0% lost mithril/crystals for scorpions/spiders is unmeasurably better than the rare resources lost with undead.
The main difference in the debate between undead units vs magical beasts comes down to time. Which one gives you the most benefit in terms of time? Consider the amount of time it takes for scorps/spiders to heal in the hospital versus the amount of time it takes to farm up enough resources to rebuild an undead army.