Skeletons
Skeleton Soldiers attack the fastest and do the most damage per second (DPS), dealing slash/dark type damage to enemies. However, their sword can only hit one enemy at a time. This makes them ideal for fighting bosses. With the upcoming arts update, they also have the most defense, so they can survive those bosses much easier than other skeletons.
Skeleton Archers attack slower and do less DPS, but they deal ranged missile/dark type damage. In addition, their arrows hit multiple enemies at the same time around their target (16 AoE Attack Ratio). Due to both of these reasons, they are ideal for fighting multiple enemies at once.
Skeleton Mages attack as fast as archers, however, they do the least DPS. They deal magic/poison damage and are ranged attackers. Their standard auto attack cannot hit multiple enemies, however, they occasionally cast an AoE acid skill that deals damage in an area and applies stacks of the Necromancer Bane debuff.
With their attribute, they provide a buff to all of your summons that make them immune to knockdowns and knockbacks, and this is extremely useful if you do not have access to a Rangovas Necklace (which has the same effect). With the upcoming arts update, they can also buff your soldiers for increased defense and your archers for increased attack speed; however, this limits them to one mage at a time. Due to all of these reasons, they are ideally only used one at a time for support (if at all).
Zombies
Zombies attack quickly and do the most DPS of all 3 zombies, however, they have very low movement speed. They deal strike/dark damage. Zombies are extremely frail compared to skeletons.
Wheelchair Zombies have a much higher movement speed than normal zombies and attack just as quickly. However, they deal less DPS. They deal strike/dark damage. Wheelchair Zombies are just as frail as zombies.
Large Zombies are extremely defensive, having a very large amount of HP; multitudes more than the other zombie types. They also have a wider attack range, hitting multiple enemies at once easier. However, they attack slower and deal significantly less DPS than both other zombies. They deal strike/dark damage.
Hexing debuffs your target and enemies in a very short range from that target. The area of effect is very small. You can optionally use the Skiaclipse Caster staff to spread your Hexing debuff easier.