Alright, let's have some fun here. I am essentially following the documentation by Wolfram, just looking at different quantities.
N[UnitConvert[Quantity["earth's gravity"]]] 9.80665m/(s)^2
Hmm, alright, fine, that's what's called the standard value for the constant g, sanctified by international convention, so I guess that's about as good as it gets.
UnitConvert[ Quantity[WolframAlpha["earth's gravity", "MathematicaResult"]]] 9.81456m/(s)^2
Hmm, fascinating. It's close, but not the same. Anybody know what the heck that's supposed to be? Is this perhaps the gravitational acceleration at the location of the query? That would be kind of nifty... Any way at all to find out?
But wait, there's more:
N[UnitConvert[Quantity["speed of sound"]]] 343.2m/s
Cool. Turns out that this happens to be (a somewhat decent approximation of) the speed of sound in air at standard conditions, but how in the world could anyone know that? The issue in this case is that "speed of sound" is woefully under-determined. I did not specify the material I was asking about, let alone pressure and temperature. The good news is, if we ask via Wolfram Alpha, we get the same answer.
O.k., so how about this one:
N[UnitConvert[ Quantity[WolframAlpha["speed of sound in water", "MathematicaResult"]]]] 1482.3846m/s
Well, alright, we specified what we wanted more accurately, and we did get a somewhat useful result. We're not sure under what conditions this speed of sound obtains, but, hey, we're confident that there's some conditions under which the speed of sound in water has that value. Of course, not knowing what those conditions are makes an 8-digit result somewhat pointless, but let's not be too picky here.
O.k., let's try this now:
N[UnitConvert[Quantity["speed of sound in water"]]] 85487.31kg/(s)^3
Come again? What on earth is this supposed to be? Anyone know?
So, the real question here is, is there any way to make such queries as the above reliable? Is there a way to find out where exactly those numbers are coming from, and what it is they describe?