You are not forced to use Viking Units....

[Loopy]

FWIW, what I have is:

Black Dragons deal 415 effective damage each, and average .2075 effective damage per leadership.

Thorn Warriors deal 2.6 effective damage each, and average .325 effective damage per leadership.

This is assuming that both units have base crit rates and that neither is beyond the +60 or -60 attack cutoff. In practice, Black Dragons will be beyond the cutoff often, so expect them to deal something like 25%-33% less damage then this on average.

Now, the real benefits come in when you add items to Thorn Warriors:

Thorn Warriors with +100% damage +1 +1 damage deal 7 effective damage each, for an average of .875 effective damage per leadership.

You can thorns even more insane, with crit and +attack bonuses you could easily double or triple these numbers. The only comparable item boost I know of for dragons is the -leadership item, which (IIRC) is -20%, meaning dragons max out at .26 effective damage per leadership unless against high level units where their higher attack can add a bit more in.

Thorns have to get across the battlefield to do anything. And they can and should be inhibited by cold, slow, traps, ice shards and anything else. Dragons will attack first turn.

Wait on first turn, let enemy move, you'll be in range of at least one enemy unit on their first turn. Your units aren't inhibited by any of that stuff unless you purposefully move into them.

Dragons can hit 2 targets at once, or anywhere up to what, 7 with its special.

Only if enemy units are positioned right. Thorns can easily hit more than 2x the damage of dragons with their base attack. And concentrated damage is far, far more important than spread out damage. Your dragons could be counter attacked by a 50% dead stack that would be 100% dead if it was hit by thorns instead.

If I may mention the Thorns special ability: After killing a target you can get an entire new stack of thorns. Permanently. Can't remember the size, but IIRC it was something like 2/3rds of the parent stack? Yeah, thats a permanent +66% damage and health boost if you need it during a long battle. Probably slightly less since some Thorns will die by that time, but still a huge benefit.

Dragons reduce the defence of each stack they hit and kill 6.5-13% of the stack per turn. Dragons can hit fire- or magic-damage stacks and basically ignore the retal.

Defense reductions won't matter much since the Dragon already has more than +60 attack in most cases. %health loss doesn't help much unless you are letting the stack sit around for several turns, if you want the stack dead NOW you want upfront damage.

Resistances and defense of the dragons is worth mentioning though. It's the main advantage of high tier units that they can last longer in a long battle. Though after +defense, Thorns do have comparable if not greater base effective health per leadership.