Alright, Nasu Time!
Heads up tho, I'm not sourcing stuff here—this is just straight from repeated osmosis and discussion on TM-General and all the Fate RPs here.
ON AGE, FAME, AND MYSTERY
The triage of mysterious factors that create a general sense of scale in the level of Heroic Spirits. Age is often the big one—as evident by Gilgamesh being the first Hero and very powerful, though what happens to pre-Gil Heroes/others from his Epic often gets debated (I can discuss as well, but it's not immediately relevant to the recent debacle). But the fact of the matter is that Gil actually is really high in all of the 3 factors due to his myth taking place at the end of the Age of Gods/his tale triggering the rise of the Age of Man which puts a far heavier weight to 'mystery', and his fame from being 'the first (complete) myth' and so on and so forth. The problem overall is that we have no scale of 'which is more important/effective' whatsoever, so it's hard to say if you can have enough of one to compensate for others (ex. High Age but Zero Fame/Mystery, etc.)
Important to note that by Mystery I'm not referring to things like "who was Jack The Ripper" (mostly), but more of the inherent 'mystery' that thaumaturgy and magic need to work. To simplify, Mystery also
generally is decreasing over time (as always, Nasu is rife with exceptions to nearly every rule).
ON 'MODERN' SERVANTS, ANGRA MANYU, AND ARTORIA
Now with the "Big Three" laid out, one more major concept that's very important is that of the "Modern" Servant. In simplest terms, there's a cut-off date (specifically for Age, not Mystery) where a Servant's abilities become worthless. This cutoff is specifically ~1900, with (as I recall) the harshest time I've heard of being 1890. Putting aside the debate about which date is relevant (birth/death) for the moment, the fact of the matter is that Jack the Ripper's canonical murder took place right at the cusp of that time—1888—with other debatable ones up through '91 still not intruding upon the twentieth century. That and considering the actual large amount of fame that Jackie the 'Stripper' (versus Fate/strange fake's Jack the Ripper, which I won't delve into) actually garnered into the modern era and that her stats aren't completely insane in Apocrypha while still being arguably the greatest Murder (because let's be serious—in the days of Uruk up until modern times, a string of killings was more like a fight/battle/war and in all honesty was plain more accepted the further back you go, and conditions make a difference. Andrew Jackson wouldn't have 200 in Murder but he'd probably have 200 in Dueling—albeit gun-dueling versus sword-dueling, etc.), the murder stat was fine as it was.
As for Angra Manyu and Artoria? The fact of the matter on the former is that he
never actually killed anyone in Nasuverse lore: Angra Manyu was a kid that was sacrificed, idealized as 'all the world's evils'. Really, his 'rank' in murder should be zero, maybe whatever an average person would have or slightly more because of 'all the world's evils', but that denomination doesn't do anything for his skills as Servant Avenger. Artoria on the other hand
could be used to make a bonus case for Jack the Ripper considering what Britain stands for in Nasu: concentrated Age of Gods-power during Artoria's time (this is why Excalibur is so crazy good and she's a top-tier servant despite being moderately more recent).
Mmmm... Eh, that seems about right for now.