1) Blood Weapon falls off too quickly.
Blood Weapon essentially functions the same as Delirium and WAR's Inner Release: you turn it on at the second slot of your global cooldown in order to buff your next 5 GCDs. While Blood Weapon suggests to be the same 10 seconds just like Delirium and Inner Release, it tends to fall short quite often before you hit your 5th GCD.
I've tried with higher SKS ending up in 2.38 secs, and still seem to miss the 5th GCD unless I REALLY focus on the timing. It seems unreasonable to be painstakingly doing this every minute of its cooldown.

2) 15 seconds is short for a DRK to retain combo.
Now that combo bonus effect falls off in 15 seconds (which is 5 more seconds than before), what you CAN do is stop a combo mid-succession and pop your Delirium early, do your 5 Bloodspiller routine and comeback to your original combo. But that means you have only about 0.6 seconds to retain your combo (1 GCD from initially leaving your combo + 5 Bloodspillers = 6 GCDS, 2.40 per GCD would be 14.4 seconds in total)
In fact, I'm almost convinced this was an accident and wasn't meant to cater for DRK's changes: GNB has the cartrige combo that only lasts 3GCDs, from which you can easily return to your original combo. 10 seconds wouldn't have made the trick, so devs could have been like "yeah we'll just throw in 5 more seconds for that", unaware of how it puts DRKs in a sticky situation.