Concern about the Aetherytes

[Enkidoh]

Also, the risk is not actually as high as you would think - before you can teleport to a location you need to first attune to said aetheryte, which... is simply not possible for an unfriendly army (read: Garlemald). This is because teleportation requires the aethertye as a beacon to guide you through the 'aetherial sea', without that, you would get lost and trapped in it forever.

There was a spell known to the Sharlayans called Flow which bypasses aetherytes, but is considered so dangerous for obvious reasons it's acutally forbidden (this is the spell Y'shtola used to allow her and Thancred to escape from the Brass Blades at the end of 2.55... and... well... that went about as well as you would expect - Thancred ended up naked in the Dravanian Forelands and Y'shtola... was lost in the Lifestream for weeks until the Senna siblings managed to get her out, using the elementals to do so. Flow also aetherically changed both forever - Thancred has permanently lost the ability to manipulate aether (and thus cannot use magic now nor will he be able to teleport ever again), and Y'shtola is physically blind, resulting in her having to consume her own aether to 'see' through the aether, which is shortening her life as a result.

Having said that, there was one occurrence of a hostile force being deliberately teleported into a building with tragic consequences, but that was actually Ascian magic and not regular teleportation available to normal people (Lahabrea in his Thancred-suit teleported Livia and her force inside the Waking Sands, completely bypassing the Immortal Flames troops guarding it outside... and even then he effectively used himself as an aetheryte of sorts to do so.).

Either way, teleportation is not nearly as useful for transporting military forces as you might think, and that is by design.