Necromancy is fun but it svcks compared to atronach summoning unfortunately. For the following reasons:
1) Relies on corpses. This means you have to kill someone first to be able to have a summon. Atronachs have no such limitations.
2) Reanimations take too long. When you summon a Dremora Lord he's already killed 2 bandits by the time the "portal" closes. When you raise a corpse you're already half dead by the time he stands up.
3) Summons turn to ash when they die. If your Atronach expires or is killed you can summon another one immediately, only cost is magicka. If it happens to your undead you have to find a different corpse because it turns to ash and is unusable.
4) Power of Summon relies on power of corpse. If you're fighting 3 draugers and 1 drauger deathlord, That drauger you killed then raised will be worthless against the Deathlord, but if you summon a Storm Atronach it can hold its own. Also the fact that you can raise Draugers from the dead, which have already been raised once before obviously, shows that my point #3 makes no sense.
5) Undead summons can't transition between zones. When you go from "Ruins Cavern" to "Ruins Crypt" your zombie companion turns to ash immediately even if you just raised him moments before.
Actually Necromancy is much more powerful than Atronach ... but at high level , you need the master spell ..
Your 1) and 2) are irrelevant since you raise your powerful summon a long time before the fight occurs , before you even enter the dungeon , and it's not advised to attempt to raise a corpse during a tough fight , it's a waste of mana , you raise the bodies before the fight or you don't raise them , you just summon Daedras or Atronach if your Thrall dies (which is very rare if you choose your thralls carefully )
Your 3) is totally wrong (except at low level ... ) , there is a spell called "Death Thrall" which makes summons Permanent , if you play on the PC , there is a mod that allows you raise even more different types of summons (like Drauger Over Lords , Briar Hearts etc ) , because in vanilla , you can't raise any NPC that is above level 40 (which is stupid ) but even in vanilla , Necromancy is much more powerful than Atronach because you can make an army of Death Thrall by giving them Summoning Staves , that means you get a group of 8 guys entering the dungeon including yourself , your 2 Thralls + their 2 summons , Your companion+ its own summon , and your dog ... You're just an unstoppable force ...
You can "Thrall" extremely powerful guys like Orchendor (even in vanilla ) or Sebastian Lort , they annihilate any Atronach in a few seconds , they use magic like masters and are almost immune to it .... Your lonely atronach would be pathetically weak next to them , even next to a normal Draugr Death lord who uses a shout and uber weapons that you carefully upgraded for him ( Atronach cannot have upgraded gear )
4) and 5) are incorrect too , your thralls will make short work of almost everything you encounter in the game (on pc you can open the console and type "getlevel" to know the level of the corpse and if it's worth raising , anything below level 30 is worthless later on ) .
The thing i agree with you is that when you do not have "Dead Thrall" , then yes Necromancy is not really powerful and Atronach (well storm atronach ) are superior in most cases , but " Dead Thrall " is the main objective for any necromancer .
I also agree that with some Thrall (mainly Daugr) , it's sometimes tedious inside dungeons because their pathfinding is either horrible or too slow , so you often have to retreat back behind them when you meet some opposition . But not all Thralls have bad pathfinding , for instance master conjurers or master vampires have decent pathfinding