Originally Posted by
fumei
I guess to me it's a question of where people think the line is drawn for what's "not faithful enough". FSN as the first work sets a standard somewhere inbetween "the actual myth" and "complete fiction", though perhaps leaning closer to the former, while subsequent work seem to either draw more closely to the middle, or lean more towards the latter overall.
Or, to rephrase, it seems to me that the parts that push people to say "this is bad" isn't necessarily a lack of faithfulness overall, but the addition of specific "unfaithful" elements.
To take Zeus as an example again, in Fate he's still the same god we know in myth, he did all the same things, and obviously didn't really change from his disposition in IRL myth. But he's a robot. That's basically the only real change here, which to me feels like it's perfectly straddling the center of faithful<->fiction, and yet it's supposed to be bad because "why would zeus even be a mecha?"
I personally think that a (sometimes quite big) degree of original fictional elements are necessary to add onto the faithfulness, because it is a fictional story and I want to see how they fit stuff into the pre-existing setting, and some stuff just have to be changed to a large extent (for example your mention of Lancelot's profile, where I'd say that personality except for literally just surface-level reading should always be fictional to best fit the character as it exists here and now, not how it fit the original myth) to do that. Of course, this doesn't apply to Altera as much because Altera is way closer to pure ficiton, merely encroaching on apparent faithfulness in order to fit her into the setting better, but then, like someone else said, the issue seems more to be with Altera specifically, not some sort of grand "OC bad" deal.