An excellent dichotomy, because...well...
There is no such thing as an actually simple caster. And there is no such thing as an actually complex martial. 5e has the most "complex" martial character (in 5.5e, probably either the Wild Heart Barbarian since your totem animal is per-rage now, or the Battle Master Fighter for obvious reasons) being dramatically simpler than the most "simple" caster (probably a blasting-focused Sorcerer? Or maybe a by-the-book Warlock of some kind?) It's frankly hard to even call any spellcaster simple, the closest being the Paladin...which is 5e's closest approximation of a true, innate caster-martial hybrid to begin with.
For my part, the heavily complex classes should reward clever play, but as a direct consequence, be frequently subject to "well, you chose the wrong situational abilities for this context, so...sorry, not much you can do right now." Conversely, the heavily simple classes should have a pretty good solid baseline of capability in all three pillars, but be inflexible with that kit: when all you have is a hammer, every problem looks like a nail, more or less. In the grey area between the greatest extremes thereof, there would ideally be classes that have a little bit of flexibility, but still mostly locked in.
The revised Champion Fighter actually isn't the worst (anymore), but it's still a bit short of the "something for every pillar" department. Had they not done the utterly infuriating design choice of making Tactical Mind eating your Second Wind uses, that would have gone a long way toward closing the remaining gap for players who really do just want The Simplest Experience, Please. And then, just as Eldritch Knight gives just the lightest taste of what a Wizard can do, Battle Master would thus be the lightest taste of what a proper Warlord class could do, with the uppermost echelons of a Warlord being moderately-high complexity--not as complex as the most complex casters (WIzards, presumably), but comparable to (say) a Cleric.
