Any ability that has a dynamic or delayed instant action is likely to be greatly affected by lag. If the server is not performing very well, you are going to see a lot of stuff work inaccurately or slow down considerably.
Possible affected abilities:
Bonespear
Dragonsbreath
Lunge
FlyingKick
Darkbolt
Web
Fireblast
Blink
Earthwall
Forcepush
Forcepull
Shuriken
GrapplingHook
IceVolley
AimedShot
And probably a few more that I just cannot remember at the moment.
There is not much you can do about it, as these skills rely on specifically timed actions in order to function. (Forcepush for instance, pushes you up, waits a fraction of a second, and THEN pushes you away.) If the server is unable to process these timed actions in a...timely manner, you will see them occur more slowly, resulting in decreased effectiveness or more easily dodgable projectiles. It is an unfortunate limitation we are forced to deal with.
However this is not much different than how vanilla Minecraft itself handles lag. You ever try to throw an enderpearl when we're at 4 TPS? It's pretty awful.
Lag in Minecraft is one of the most aggravating experiences I've ever had, and Mojang is unlikely ever going to handle it any better. Meaning it's here to stay for HC abilities as well.
