While an average level 1 unit, the necromancy skill makes the skeletons a great choice for large maps with a lot of wandering creatures, which can be harvested.
Disadvantage: On tiny maps occupied only by castles, where the majority of the battles happen during the first two weeks, updated gremlins or sprites would be a better choice since they are better suited for castle conquests during the first weeks than skeletons.
Updated harpies are masters of guerrilla attacks as the enemy does not retaliate and the harpies return to their original position unharmed. If combined with mass slow, melee units can have a tough time to take down the harpies. If you dislike loosing units while cleaning the map, this is the creature for you. It was voted to be the second most favourite level 2 unit.
Disadvantage: While harpies are awesome units for map cleaning and castle defense against weak opponents because of their low attrition, harpies are lousy units for final battles where big unit losses have to be expected. If you expect early battles between the main heroes in the open field, wolfs with their impressive damage ability would be a better choice.
Each castle should have at least one shooting unit. Upgraded elves shoot twice during the turn, making their upgraded version highly desirable. Elves were voted to be the favourite level 3 unit.
Once the count of upgraded vampires reaches ~20 units, they are awesome beasts for map cleaning as you may expect minimal losses due to their ability to resurrect from the flesh of their enemies. Rated the most favourite level 4 unit.
Disadvantage: Just like harpies, vampires are not the best units for the final battles. Furthermore, it takes two weeks to grow the necessary number of units from a single castle.
Upgraded pits can (or could, if they worked also on undeads) raise ridiculously expensive vampires in vast quantities from the slaughtered skeletons. Finally, you do not have to worry about skeleton losses during map cleaning as the lost skeletons get converted into vampires!
Disadvantage: Just like harpies, elves and vampires, this unit has to be upgraded to benefit from them fully.
To compensate the need to upgrade almost all lower level units, wyverns do not have to be upgraded to be of some use. Furthermore, wyverns' dwelling has very low prerequisite requirements (just a level 2 dwelling building), making it possible to recruit the wyverns during on the first day. Since they are also fliers, they can be of great use for early castle sieging.
Disadvantage: Champions, black knights and nagas are all beautiful creatures. But no other level 6 dwelling can be build on day 1. If we went with any other unit, we would not be able to recruit level 7 units on day 2 of the first week.
While firebird is not the best level 7 unit, the fact that the dwelling is one of the cheapest (the second behind titan's), the unit itself is inexpensive, the weekly growth is above average, it is one of the fastest units in the game giving you the opportunity to apply mass haste/slow and decimate the opponent before he gets to the turn, and fire immunity allowing you to embrace Armageddon strategy (with the help of a support hero to store raised skeletons) make it a pretty decent choice. It is the only level 7 unit whose parameters were down-tuned in Horn of the Abyss. Furthermore, firebird dwelling building has only one prerequisite - level 6 dwelling building. Hence, if you play on normal difficulty, you just need to get 5 wood (or better, set the starting bonus to resources - wood and stone, and you do not have to worry about getting any additional resource or gold) and on the second day you will have 2 firebirds, 2 wyverns and 3900 gold in the pocket to spend it on whatever you want to (based on the situation you may want to keep it for capitol erection, or spend it on another hero, 8 harpies and 6 skeletons).
Since all the units would be from the same castle, there would not be morale penalty from mixing several units together. The castle would be usable for blitzkrieg strategy during the early phase of the game if played on normal difficulty level thanks to wyverns and firebirds. But the castle would also scale to long games due to the ability to harvest skeletons, which would be continuously converted into vampires. And because upgraded harpies and vampires simply do not die during the map cleaning, you may go into the final battles with vast counts of skeletons, harpies, and vampires. Furthermore, firebirds and Phoenixes are, contrary to many dragons, immune to Armageddon spell. And because Phoenixes are the fastest units in the game, you are almost guaranteed, that you get the opportunity to cast Armageddon before the opponent gets the opportunity to do anything. Hence, Phoenixes can be in some situations used for dragon slaying without any loss.