There seems to be an issue with actual game event happening and chase chains when it's on 2x. Often times, if you cast near the end or even in the middle of a long chase chain on 2x, your newly cast skill will not happen immediately after the chain happens (this is, ofc, when it SHOULD be the next one. Higher init/order skill is another story).
Other than that, with really good connection, on 2x usually whatever your cast at most 2-3 moves after you cast it, usually around 1-2. Still, it doesn't allow you to do something "immediately after" something else, such as neji acupuncturing immediately after the enemy who have cleanse on their normal "attack" so that the acupuncture *s for next turn instead of being removed.
This is an even bigger problem if your team have chakra generators like jigokudo or orochimaru. If the chakra comes at end of turn due to a clone attack(yes, clones can kill, often other clones) or something, it's almost impossible to get the extra skill casted at 1x speed and certainly impossible on 2x. Fact is 2x affects different compo differently, it severely hurts builds where timed action instead of "spam at start of turn" action are important. It's not a fair option in live pvp.
That said, it won't hurt if every player have a tick box somewhere to allow for 2x battle in PvP and if every participant in a fight (2 or 6) have it ticked, the battle can be locked at 2x instead. When everyone is a "spam skill at start of turn" sort of build, that will work out. But for all other fights, 1x it is.