Necropolis discussions.

Actually, skeleton archers aren't that scary; gremlins are noticeably stronger, so there's no need to be particularly afraid of them. IMHO, the undead's dangerous units are vampires (especially in their upgraded form), liches in reasonable numbers (6 liches aren't game-breaking), if those liches are even built... and the banshees; they are powerful. In general, after playing the undead faction in HotS and in single-player, one can say that the undead's strength lies in their cheap resurrection magic. Level 2 and 6 mana allow for easy map clearing at the beginning, providing a good start. Their weaknesses are the generally low initiative of the army and the lack of clearly defined tanks – monsters capable of absorbing mass damage.