Modern vs Medieval, what time is it?

[Enkidoh]

Y'shtola isn't flubbing her spells though. Flow is working exactly as intended, which is to say teleporting the target "anywhere but here" with a chance of ending up in the aetherial rather than physical realm. It's explained back in Heavensward (when they first worked out that she cast it) that normal teleportation spells require incantations and take some time to cast, but Flow is instantaneous - which is why she's used it as an urgent last resort.

Although that is correct, it's not just the incantation time required by normal teleportation spells that seperate it from Flow, what makes Flow so dangerous is that it bypasses the whole point of using aetherytes and simply relies on the will of the caster to get themselves through the Lifestream to their destination. Which 9 times out of 10 meant the caster just got themselves lost in the Lifestream with no way out, as Y'shtola has done twice now.

Normal teleportation uses the aetheryte network as a beacon to guide you to your destination, Flow has no such lighthouse. With Flow, you may end up at the place you want. Or you may end up somewhere completely unexpected malms from where you wanted to go. You may not even arrive, being stuck in the Lifestream forever, effectively killing yourself as your corporeal form disappaites in the aetherial sea.

And if you are lucky enough to get out, you may find your physical form has been harmed by the experience, being completley altered. You might find yourself sans clothing, as naked as your nameday. Your body's aether might be damaged preventing you from casting magic permenantly (to the point you won't even be able to teleport normally anymore). You might find a physical sense like your sight or hearing permenantly lost.

Accordingly, the spell was considered so dangerous for these reasons Sharalyan banned it (Matoya alone gave Y'shtola one hell of a verbal lashing over it, so horrified was she that her pupil was so reckless in breaking such a taboo), and, given that we now know that it was the Ancients who originally invented aetheryte technology, even they probably thought such a teleportation technique was pretty much a 'blind jump' (or, more appropriately, a 'blue jaunt' ) that was bound to just be too risky to bother with, and so set up aetherytes and 'conventional' teleportation as a far safer alternative.

[LineageRazor]

and, given that we now know that it was the Ancients who originally invented aetheryte technology

Hang on a minute - when was this confirmed? I don't doubt that it was, but I don't remember reading anything to this effect.

I figured the Ancients just bamfed around like the Ascians do when they want to get someplace in a hurry.

It would explain, though, why Emet included an Aetherite in his recreation of Amaraut...

[Enkidoh]

Hang on a minute - when was this confirmed? I don't doubt that it was, but I don't remember reading anything to this effect.

I figured the Ancients just bamfed around like the Ascians do when they want to get someplace in a hurry.

It would explain, though, why Emet included an Aetherite in his recreation of Amaraut...

That's what I was referring to, not only the aetheryte in the middle of Amaurot, but that an identical one is also in the middle of the ondo encampment (which the ondo settled because it was an Amaurotian ruin - both are in a unique style that screams 'Amaurot', and the ondo one to me is unlikely to have been something that Emet himself just gave to them (they don't seem to use it themselves at least), with the only remaining logical explanation is that it was always there and thus had somehow survived the Terminus and Sundering, and twelve thousand years under the ocean intact and fully operational (given how hardy Amaurotian structures seem to be, this is not surprising), and that the knowledge to make the technology has just been handed down by the Ascians to various societies on the Source and the Shards over countless millennia (we already know that was actually the case - the Ascians were teaching the beastmen on the Source how to make aetherytes as part of their summoning of Primals, yet the ondo's one is clearly not 'beast made', it is the exact same design as the one in Emet's reconstruction of Amaurot).

Also lets not forget an old piece of lore from 1.0 that explained why Eorzea was first settled was because of the aetheryte network already being there, made by 'hands unknown', so maybe the aetherytes seen in The Tempest at the Ondo Cups and Amaurot are the original design, with the others just variations (the other aetherytes found in Norvrandt seem to have a clunky 'primitive' look to them completely different to the ones on the Source and the ones in the Tempest, as if imperfectly copied by people who don't understand the concept or technology fully).

But yeah, there is no 100% confirmation about that so take my comments as 'highly probable based on observations but not yet proven conclusively'.