Jordis is not a very effective follower. None of the Housecarls are, actually. Hirelings are the good ones.
I have to disgree with you. Your follower's effectiveness depends on the level your character is at the first time you encounter them. For example, a lot of people think that Lydia is an inherently weak follower because she's usually the first follower you get; and often times you get her when you're at a low level. And since followers don't level up along with you, it's only a short time before she becomes ineffective. The same goes with any hireling. If you stroll into the Drunken Huntsman for the first time at level 3 and hire Janassa, then she'll be at level 3. If you encounter her for the first time at level 30, then she'll be level 30, etc.
All Housecarls can level up to 50, thereby making them potentially more effective than most others. J'Zhargo can level up to 81 so he's the most powerful one in the game.
To level followers you need the Wabbajack.
1. Get the Wabbajack
2. Have your follower with you and then dismiss them ("It's time for us to part ways").
3. Fire the Wabbajack at them until they become a chicken, rabbit, or mudcrab
4. Wait for them to return to their normal form and then they'll be at your level provided you're not above their level cap.
5. Then tell them to follow you again and now you have a follower that's your level
6.. IMPORTANT: Save before you do this because often times they'll be killed by the Wabbajack. If they turn into a pile of gold or a sweetroll it means you've killed them. So reload and try it again. If you kill them-kill them, reload and try again.