I've done just this but it still feels sucky.
One attack with GFB at 5th level, just isnt the same as swinging that greatsword twice. Moreso on a Paladin thanks to 2 smites being better than one.
Considering a Paladin can dump Dexterity and Intelligence (heavy armor), and still need Strength to wear that heavy armor, I dont see the MAD as being an issue for them to justify the dip. Ditto Bards, and ditto Sorcerers (each for different reasons).
Compare a Paladin (default array, +2 Charisma at 4th) Vuman at 5th level: Str 16, Dex 8, Con 14, Wis 12, Int 10, Cha 16.
If he's dumping Strength to instead focus on Charisma via a Hexblade dip, he's not using heavy armor (or likely even medium armor) due to encumbrance, meaning he needs a good Dex now. His AC is likely worse than that of the above single classed Paladin, he misses out on extra attack, and likely only has a hit bonus of +1 higher than the Strength based single class paladin to make up for it.
He has access to shield, a recharching 1st level slot for smites, and a better ranged attack with EB, but that's really more of a fallback option; his main schtick is getting in someone's face and smiting them dead, which the single classed Paladin simply just does a whole lot better.