For me, Darkplaces is an engine with Quake roots mostly fit for custom projects, while retaining a fair bit of Quake-compatibility. As far as I can tell, if Darkplaces breaks Quake or its mission packs, this is still considered a bug.
Note that Darkplaces contains a few cvars to control fixes for (in most cases) clear bugs or oversights in original Quake. Just search for "gameplayfix" on https://icculus.org/twilight/darkplaces/readme.html
- the downside is that content dependent on the original quirks may break.
The renderer has changed enough that getting it to look "quite exactly" like the original renderer is not easily in the cards. For instance, dynamic lights won't shine through walls, which is of course "correct", but on the start map for mission pack 1, the original quirk was used for a "running light"-effect that won't work show in Darkplaces. It's things like that some people don't consider "faithful", but then again it depends a bit on your reference point. For instance, I consider GLQuake (albeit from id) to be even less faithful (missing fullbrights and overbrights).
One neat thing in Darkplaces is its networking capabilities. Want a single-player-capable engine without the need for being "super-faithful", but also join NetQuake or QuakeWorld servers with server browser and that stuff? Well, then you could do worse than Darkplaces, IMO. You can also check out FTE, of course.