You have to get 13s in 4 stats to use all those classes, plus you miss out on a higher crit range. The extra attacks are nice though. Also the sneak attack and GWM won't work together.
Correct. I didn't put the entire build in, so sorry for any confusion. Great weapon fighting from pally and pact of the blade are part of the build as well.
5th lv lock gives you access to Eldritch Smite, and as you pointed out, 3rd lv spells, so more dice. Only 2 uses per SR, but if you're in a party that SRs often, then it can add up. Plus it's force damage, which is nice.
Half Elf
Paladin (Oath of Vengence) 6
Warlock (Hexblade) 5
Bard (College of Whispers) 9
Feats/ASI: Pole Arm Master, Elven Accuracy, Great Weapon Master, Cha/Cha.
This is very much a once per rest build, but can dish out some nasty crits. Can work well in boss fights, but falls short in hord or...