Aela, like most followers that start with a bow, won't use a better bow if you give it to them, which puts her at a disadvantage. And since she's one of the first characters you meet, she spawns into the world at a low level (and followers don't level up).
It is possible to make her an essential character after the Companion quest line. If you start the hunt for the totems (which you get from asking her for a job) she'll be come essential again and won't be able to be killed until you get all three.
As with any follower, if you use the Wabbajack to turn her into an animal, her level/stats will reset when she reverts back to match your own, up until level 50 I believe. Just know that you can't Wabbajack a follower that is essential, so you would have to do this before the Totem of Hircine questline if you want her to be not able to die.