How good would this mc be?

[Spacehamster]

11 levels eldritch knight, 7 levels oathbreaker and 2 levels warlock, 3 attacks with STR + CHA damage, great saves, 2 short rest slots for keeping hex up all the time and decent amount of spell slots for smiting, plus 2 combat styles, action surge and other goodies, at the cost of 1 less asi. INT can still be kept low with taking save/roll less eldritch knight spells, making it as stat dependent as a normal paladin.

[bid]

You don't get your 3rd attack until level 18 and you lose your pally 11 improved smite. I'm not sure pally gains much from EK after level 3.

[MaxWilson]

11 levels eldritch knight, 7 levels oathbreaker and 2 levels warlock, 3 attacks with STR + CHA damage, great saves, 2 short rest slots for keeping hex up all the time and decent amount of spell slots for smiting, plus 2 combat styles, action surge and other goodies, at the cost of 1 less asi. INT can still be kept low with taking save/roll less eldritch knight spells, making it as stat dependent as a normal paladin.

It's not bad. Somewhat melee-oriented, and not very many spell slots compared to a normal paladin (4 3 2 plus 2 warlock slots, compared to 4 3 3 3 2 for a normal paladin or 4 3 3 3 2 1 1 for something like a Paladin 9/Sorcerer 9/Warlock 2), and you lose out on Improved Smite and 3rd-5th level spells relative to a normal paladin. The main at-will advantages you have over a normal paladin are a fighting style and that third attack per round, so you better make it count more than Divine Smite would have, probably by taking GWM. Action Surge is a nova benefit and so is the second fighting style, and access to Shield is nice but could be had in other ways e.g. Sorcerer.

The Eldritch Blast + War Magic synergy is interesting but doesn't benefit from Oathbreaker unless you also take Crossbow Expert to use it in melee.

Will be a bit MAD and short on ASIs.

Overall I'd say this would be a fun PC to play, if you're into the Oathbreaker fluff. But consider Eldritch Knight 8, then Bladelock 12 as an alternative. Similar fluff, no smiting, but it develops earlier (3 "attacks" per round by level 9 w/ War Magic), has a lot more damage, and the round-by-round damage maximum is identical: Eldritch Blast plus bonus action with Lifedrinker. Plus you can Mass Suggestion or keep three ghouls on a leash at all times (Create Undead), and you can Fireball and Armor of Agathys with your 5th level slots.

Argh, that sounds so fun that for the first time I am truly tempted to play a Warlock. :)