Well depends, if the enemy is a naked human - yes it would make sense for him to die with one shot. (he probably won't be much of a boss...)
But if the enemy is some kind of mutated monster, heavily modified human, or just someone wearing especially made very powerful armor and shields - 5K HP would make sense.
Even a regular Human boss I expect to not die to a single headshot. That doesn't mean I want a bullet sponge either, but it shouldn't go down in 1 hit however you want to explain that. In Mass Effect, specially made armour with advanced shields sounds like a good way to do that.
What I mostly want is for the fight to be interesting. If the boss dies in 1 hit or if it behaves like the trash mob I fought up until that point except with higher stats, then I'll not find it a terribly interesting boss fight.
Sometimes making an interesting boss fight means having a mechanic that the player can't access through any gameplay choice, which by definition is no longer symmetrical gameplay. I'm okay with that though, because it means more interesting boss fights.
Eww...
I just took a quick look at some comments on metacritic, and a lot of people seem to think they've ripped out a lot of the RPG elements and made it more of an action shooter. Also, there is a lot of criticism about them having voiced the PC.
A dear friend of mine who is a major fan of both FO and TES and plays both series repeatedly picked up FO4 on release day. She hasn't said a lot about it - she rarely praises or criticizes anything - but she did tell me she felt she was about done with it during her first playthrough.
I can only hope Bethesda will return to form going forward.
ETA:
Yeah, that was the other huge reason why I've not been eager to purchase it.
To be honest after Skyrim, I'm not surprised that they pushed to remove skills and other things generally associated with "RPG elements" from Fallout as well.
Under normal circumstances I'd be annoyed by that, but Bethesda has the horrible habit of allowing a perfect character these days anwyay. In Fallout 3, even without Broken Steel DLC that increases the level cap, it was possible to max out every skill. It's no longer a character building choice at that point when you can choose everything all at once.
Bethesda's biggest problem with the storyline is they're trying to be BioWare, but they aren't good at being BioWare. They're good at creating a world with lots of interesting lore to set us loose in to go explore and have fun in a sandbox while we RP our characters and eventually mod the game to exactly the way we want to play it.
Skyrim, Oblivion, and Fallout 3 all did a better job of this because they allow you to completely ignore the main quest after the opening tutorial areas(which can be modded away with alternate start mods) and dialogue relating to it doesn't keep popping up unless you actually do those main quests. Fallout 4 so desperately wants you to do its main quest that it makes it difficult to avoid, especially given that features like settlements are tied directly to it.
If it weren't for mods, I would be going back to Skyrim despite liking the post apocalyptic sci-fi theme better than medieval fantasy. It's simply the better base game, imo.