Answer: pretty good, but it's missing some systems like monster morale checks & morale, or the old D&D approaches to evasion, getting lost & such. My own 5e campaign is a pretty old school sandboxy thing and pretty well all the published material I use is either original old school (Caverns of Thracia, Wilderlands) or OSR (Dyson's Delve, O5R Liberation of the Demon Slayer). I've had an easy time filling in the gaps; eg I already use B/X style Morale rules in all my games anyway.
The 5e PCs IMC aren't fragile/disposable, but then old-D&D PCs really aren't either after 1st level. It's really 3e where high level Fighter types routinely drop like flies. The spell/magic rules in 5e really limit casters, in ways that are different from old school D&D but the effect is similar, warriors & wizards need to work together, no 3e/PF god-casters and their non-caster mook/grog buddies.
Power progression is different from any prior version of D&D; enemy numbers matter hugely, and a squad of soldiers is a bigger threat than a manticore - I really like this but it feels a bit more like non-D&D RPGs than any version of D&D. It definitely gives that "heroic not superheroic" feel - which I guess resembles low level D&D, but 7th level old school D&D PCs can generally wade through hordes of foes more easily than my 5e group can.