Hideous Blow has been a fantastic time for me over multiple characters, though quite tedious, certainly not as strong as blastlock, and not on-par with its melee counterparts in PvP.
In an effort to avoid buffing their overall power level, I am suggesting that hideous blow warlocks are given 2 hideous blows per round once hitting level 28 and having at least 1 epic pact regardless if they have haste up or not, and/or hideous blow ignores a (if possible) percentage or entirety of concealment. However, haste does nothing towards giving them more EBs.
The reason for this is - based on my experience - once your haste gets knocked out in PvP you are near hopeless:
1. You're attacking once per round. If the enemy has some sort of concealment you aren't getting much of anywhere fast enough.
2. While sitting in expertise or IE your hideous blow damage is reduced by 5d6 and 10d6 respectively. This leads to one of the theoretically better damaging hideous blow builds doing 91 and 73.5 damage on average; this damage assumes they have no resistances. Any reasonably well-built melee build will outpace you so quickly, especially once your haste is brought down.
3. One EB per round = one chance to crit. At the required 20 to crit that feels improbable during a PvP fight.