So, I got the quest from the Companions to retrieve a wealthy family's heirloom from a giant camp. I was only level 11 but I figured I'd give it go and see if I could be sneaky, which isn't a specialty of this character. I make it to the cave in the camp and sneak inside. I see the chest with the heirloom in it but a giant is just off to the right. Lydia "volunteers" to distract the giant by beating her face off its club while I run down, loot the chest and sprint back out. Lydia does not come back out with me. I make it back to Whiterun without her feeling rather awful and contemplating a reload to think of another option, buying conjure flame atronoch and using it as bait maybe. I take a quick nap as I got back crazy early a.m. and when I wake up...Lydia is back! Thats a good housekarl.

I knew she was 'invincible' unless you kill her but I wasn't sure she would make it back if I left her for dead somewhere.

I sacrifice Uthgerd the Unbroken to a god and in turn got a free for all and later kill some more folks and then some nice Armour

It looks like this game needs badly some sort of henchman syndicate

she isnt invincible. I have lost her to giants a couple times when i was fighting a seperate giant. If they attack quickly they can kill her while she is kneeling before they turn their attention to you.