The only plague I was quite confused by was the slimes. For being so pathetic, they did a lot of damage.
The cockroaches were extremely understandable, the problem with them wasn't that they were destroying towns or that the soldiers couldn't kill them, the problem was that even a small bite could cause major disease. And that in the sheer number they couldn't kill them fast enough to make a major impact.
Troglos and Giants were more than understandable, again it wasn't that they were too much to handle, is was more than they were on par with the strength of humans and the humans really didn't want to go to war with them.
Golems - obvious
Alien Beasts - Obvious
On a note about the slimes though: One must realize that that far south area of Valorium got almost no support whatsoever, so that could have been a problem as even one soldier lost may have been huge to them.