To clarify, as I’ve explained this point several times on discord already, the practical impact would appear to be very low. As you explain, the talent specifies that it deals damage once and Bladestorm ticks once per second, giving us two basic scenarios:
- The Bladestorm proc is an instantaneous background action, in which case there is no loss of control whatsoever; at most just a little spinny animation and immediate damage.
- The Bladestorm proc is a 1s channel, which is probably also hasted. Because it procs from Bloodthirst/Overpower, this would allow it to channel during the GCD, which means the actual loss of control is functionally irrelevant - you can still auto-attack or move during Bladestorm, and now you can interrupt/stun too!
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- The worst case scenario is you may want to Leap immediately during that GCD, which should be a fairly niche situation that can always be accounted for by simply not pressing BT/OP immediately before you want to leap; it’s also possible that a /cancelaura macro could avoid this issue, assuming Hurricane won’t stack with such a short and potentially frequent proc or is just removed entirely in favor of higher direct Bladestorm damage.
So in either case, this is probably a non-issue, and it seems very likely that the developers designed it this way specifically to avoid the proc impinging on the normal rotation. The more accurate observation would be that Unrelenting Onslaught CDR results in hard-casting Bladestorm more often, but we still don’t know how often it will be - there are definitely unrevealed baseline class/spec changes to come.