To be fair, Paladins have the same problem. Starting with 2 levels gives you heavy armor, lay on hands, and divine smite. There's no reason for a martial Bard not to start with 2 pally levels since there's no alignment requirement for those classes anymore.
Yeah, you could also claim that getting rage with a single level for melee characters or some other combo is powerful, but the problem with Hexblade is that you can focus on a single attribute to control both casting and fighting and social interaction. Its a questionable build when it comes to balance. By itself its fine, but again, it suffers from a very poor lore translation into rules for what it's meant to do.
I did a enemy NPC for my players - 8 oath of glory paladin, 1 hexblade and 3 swashbuckler rogue, ST14/ DX14/ Co 14 / IQ 8 / Ws 9/ CH 20... Initiative 7, +9 to attacks and +7 damage, saves st/dx/co +7, iq +4, ws +8 and ch +14 (+5 for all allys within aura), casting +9 to hit, dc 17 - all of that without taking any gear into account... movement 40, smites, sneak attack, cunning actions on top of heavy armor+shield and shield spell... the single level of hexblade is what pushes this build beyond because the charisma 20 affected so many things. You will rarely see a paladin at level 12 max out charisma and still dominate without that multiclass dip into hexblade.
