A little fun detail about RDM is that it is surprisingly more lag-unfriendly than even NIN.

RDM's dualcast is ultimately a buff that applies after your cast from an answer from the server. If your button press beats this answer you will start hardcasting your second spell instead of insta-shooting it.

BLM on the other hand is much less affected by lag on its core gameplay. At most you'll be unable to slide-cast at the 80% cast bar completion.

Ultimately its just a factor that matters only if you tend to be lagging.