The HP of one of my explorers is getting dangerously low. He still has 2 stimpaks (and is below level 10, so each use would heal alot). I wonder if he is 100% safe as long as he has stimpaks, or if he can be instagibbed (dying from a high damage hit even though he has healing items).
From the Exploring wikia page:
Outfits increase their statistics and ability to survive, weapons improve their chance to defeat enemies, while Stimpaks and RadAway ensure that they can heal damage and remove any radiation they absorb.
Also, from this answer,
There is no way to force them to use them, they will just use them on their own until their stock you give them when you send them out goes away.
From these posts as well as personal experience, vault dwellers will not die before using all of the Stimpaks you send them to explore with.
Stimpacks heal about 50% of a dweller's health, regardless of level or their maximum health.
While exploring, dwellers will use a stimpack whenever their health drops low enough to use it without wasting any. So, half health.
Therefore, in order to die before using all of their stimapacks, an exploring dweller would have to take more than half of their maximum health in one instance of damage, killing them before they had a chance to use a stimpack.
Considering how small each instance of damage suffered in the wasteland is, I believe that this is impossible, although I don't have hard numbers to Science! for you.
Source: experience playing the game, https://www.reddit.com/r/foshelter/comments/4c4m46/everything_health/,
Apparently, it’s possible. It has never happened to my dwellers, until now.
As you can see from the screenshot, the dweller still has plenty Stimpaks and Radaway, but she still managed to die. When I revived her, she had her HP bar 100% full of radiation. See the last entries in the journal, too.
I have no idea why she didn’t use the Radaway when she had 24 units left.
And she’s has pretty good SPECIAL too: