Level 8 - :) or :(

Do you think that Undead are stronger than Knights or Warlocks?

Well, in a 1-on-1 fight, they definitely have the advantage ;)
In my opinion, Undead is one of the strongest factions at level 6.

Isn't it an imbalance that a Bone Dragon is equal in strength to half a Green Dragon?

First, there are 3 Bone Dragons. Second, Bone Dragons aren't twice as weak, maybe 30% weaker. Third, Bone Dragons inflict Curse and Grief! :)
They are no worse... and under certain circumstances, they might even be better.

Again, it's rare to reach day 100... but I think that 300 peasants on day 100 isn't such a large army. It's better to gain experience from them.

When you have money, of course, you buy everyone too :)
But if the enemy also buys everyone, then they won't have room for an additional army either.
And then there won't be enough space in the eighth slot... should we make a ninth? and then a tenth?

Then why is WoG more popular than the original?

It's not more popular.

It's long overdue to diversify the races somehow.

They are self-sufficient.