Edit: Necromancy, already made my point on this 4 years ago.
Since then, Sage Advice has
ruled that you can't make a sentient or artifact weapon into your pact weapon. Asked whether this was for flavor or balance reasons, Jeremy Crawford claimed it was "both," but did not explain how it would be overpowered (nor, for that matter, did he explain the flavor reason).
Considering this was pre-Xanathar's, when bladelocks were competing with beastmaster rangers for "least effective martial class in the game," I'm not buying that they had a compelling balance reason. At all. Even now, it's hard to see why it's just fine for a paladin to have a sentient weapon but the warlock can't.
Yet another Sage Advice ruling to toss on the trash pile.