So, I'm a conjurer/illusionist. I'm level 47 currently. I mainly use Dremora, and I truely think the Atronachs are terrible. So I tried Dead Thrall. I went and got
but her body was too powerful. Fine whatever. I then used the Atronach forge to summon a Dremora and it was too powerful. I then did Boethiah's quest and he was too powerful.

So now I'm doing the Orc quest to try and get Chief Yamarz. I really hope his is thrallable. My question is why can't I do any of this? Why is it that Dead Thrall caps at level 40? IS there any point to that? Don't NPCs cap off at level 50 anyway? Why not add those extra 10 levels?

Also, what are some good thralls that I CAN get? Remember I'm level 47. I tried looking online, but every one I've tried already is just terrible. I have Velehk Slain and he dies right away in battle.

Thanks. This is really starting to bother me because you can't play a real Necromancer when the only spells that work make the bodies turn to ash.

If you are lvl 47 and the cap is 40, don't try named/leveled NPCs, since they will be too powerful. Try generic enemies. Strong Forsworn, Bandits, or Vampires are possible choices.

My thralls do just fine and chief yarmaz is thralleble. Galmar stone fist makes a good thrall too and he doesn't moan which is a plus.

There are plenty of very potent thralls available, the ones you mentioned like the leader of the dark brotherhood aren't available for thralls, which is fair enough IMO.

Here's a link to the elderscroll wiki, it gives you a table of potential strong thralls: http://elderscrolls.wikia.com/wiki/Dead_Thrall

Here's a link to uesp, detailed list of potential thralls and tips: http://www.uesp.net/wiki/Skyrim:Dead_Thrall

Chief Yamarz is a very good choice for a thrall, he's always under level 40 and has [censored] loads of health. You have to be the one that kills the giant in the quest though, then kill him yourself, otherwise you will get the version of him that dies in one hit to the giant.

The best thralls in my opinion (disregarding using crafting to overpower archers and dual wielders) are the volkihar vampires (level 48 but necromage makes it possible) and some of the new dawnguard vampires. You can do a prison glitch to upgrade your thrall, it will upgrade the thrall one tier. Volkihar vampire > volkihar master vampire. Look it up if your interested, I'm not sure how to do it.

Edit: just had a read of the pages, it seems you can use the atronach forge to summon a random dremora. So you can get a level 36 dremora markynaz that knows incinerate with the augmented flames perk, that will absolutely destroy enemies. Might have to give that a go. They also know close wounds, steadfast ward and ironflesh, so they should be able to take a beating.