/castsequence was never meant to be used in situations when you mash a button. In situations of high lag, it can and occasionally does stutter on an ability, casting it 2 or 3 times before moving on to the next.
Since the problem with
reset=15 has already been explained, the alternative to manually reset the sequence with
reset=mod makes it operate no different and perhaps more glitchy than using a
/cast macro with the mod condition.
Back to the base of the thread,
/castsequence never was a method to cast on reaction to a CD. The fix Blizzard put out was the "fallthrough" that happened when stringing
/cast commands together in which if the first failed for any reason, including cooldowns, it would fall through to the next
/cast command. Now macros are processed to take the GCD into consideration. This means when dealing with abilities that trigger the GCD, the macro will only analyze if the first can be cast and ignore the rest regardless of individual CDs.
For example:
Code:
/cast Chi Wave
/cast Jab
Prior to the fix, this would've cast Chi Wave if it were off CD and Jab if it wasn't. Now,
/cast will only cast Chi Wave and ignore Jab if both are on the GCD.