Lich King is def powerful enough to drive the Burning Legion back through the portal. LK is not strong enough to take on the entire legion. LK has limits but they are high enough that they are almost considered a god-tier. (Arthas literally played with us the entire fight, when he grew bored of us. He 1-shot us.)
If we are dealing with dreadlords and their command of the dead. LK def overpowers a dreadlord's will. In W3, Ner'zhul lost control of them due to the frozen throne being cracked and so much power escaping or he willingly let them control them in the Burning Legion's name.
Besides we are dealing with more then just a war. This is the equivalent of a Tiger vs a Bear.
One side gains from defeat of enemies and resurrecting them to serve their will. The other side gains from souls being driven towards their soul machine.
One side can literally shit out building as of legion and have space ships. The other side has several necropoli and can actually summon their structures slowly but surely. (And several more being built as of WOTLK)
Necropolis take quite a beating to take down. We haven't actually crashed a necropolis yet if im correct but I know we haven't destroyed a spaceship either. (We have seen one necropolis crash due to malfunctioning and one crashing due to Drakari Trolls.)
We know there are probably countless legion ships.
If we have control of all Necropoli seen ingame it would be about 13. This includes all named ones, unnamed ones, and WOTLK launch event ones.
I would also love to mention some other airplay.
As of now, Legion has Fel Bats, Abyssals, Doomguards, Terrorguards, Infernals ( and maybe nathrezim due to their wings.)
Scourge Forces has Frost wyrms(including all varities such as emberwyrms and blackwyrms), Gargoyles, Nerubian Flyers, undead gryphons, anything thats dead and can fly......
This is something that actually interests me. Mainly because I want to see a bone chicken 1v1 a fel bat.
I could go on but i've already spend around 20 minutes writing this post.