I don't see a problem with this personally. A Star Pact Warlock is the most likely to worry about both Cha and Con abilities for accuracy. One Fate of the Void boon allows a 16 Con and Cha Warlock to be just as accurate as Fey or Infernal Pact Warlock with one 20 Cha/Con ability for just one d20 roll though with 2 less damage. The hard part is getting that boon when you want it, as you have to have a precharge Rod of Harvest and spend a Minor Action to use it, or hit and attack someone with subpar accuracy.
While a Star Pact could get one major boost to accuracy on one attack, trying to convince allies to not to kill all the minions in one go so you can stay accurate can be a problem.
I'm surpised there hasn't been a call of OP on the Fey Pact Misty step. Its only a 3 range teleport at level 1, but by the time the PC gets Improved Misty Step, Eladrin Armor(Chainmail)+3 and Eladrin Boots its no longer a 3 ranged teleport. It turns into a 9 ranged teleport and later a 10 ranged after the armor goes to +5. A teleport that can be stored in a Rod of Harvest to be used on a Minor Action.
Strangely, mass killing Minions for Pact Boons actually seems to be more of a waste then a boon. Star Pact gets insurance on one d20 roll it hits, but looses the benefit for further attacks later on. Especially with all the Sustain Minor/Standards a Warlock can get on his Daily abilities that require attack rolls again. Fey Pact receives multiple teleports, but later looses the option for easy movement when a non-minion approaches him. Infernal Pact before his Paragon Path doesn't receive any benefit from the mass killing, but with 1 minion going down a turn, he always has Temporary HP to absorb damage with.
At best, 2-3 cursed enemies dieing between turns seems to be best. 1-2 for Pact Boon benefits, and 1 to store into a Rod of Harvest.