Tzeentch – Heavy focus on ranged combat and spellcasting units are protected by barriers (i.e., armor that gets restored over time as long as the unit isn’t taking damage).Nurgle – Pure melee with spellcasting options (including healing) high HP and defense, but egregiously slow movement speed.
While it’s true that each faction boasts strong units, their rosters tend to fill a certain niche, and you’d find that there are gaps that need to be rectified.
“Monogod” refers to the daemon factions that follow one specific deity: Khorne, Slaanesh, Nurgle, and Tzeentch.