It should generally apply to multiclass characters, but to expect any character of X levels to be as effective as any other character of X levels at every level is just unreasonable. Character concept has to factor in there somewhere, and we have to allow for the idea of bad concept. Some concepts simply can't work very well, because they are too scattered and unfocused.
I don't think that's the issue. I think it is a spellcaster versus martial class divide for whether combos multiclass effectively out of the box.
Multiclass concepts
Spellcasting:
Concept 1: Gandalf/Elric/Dilvish the Damned type. A wizard who is good with a sword. Gandalf is not as good as Aragorn or Legolas or Gimli in combat, but he can hold his own plus do neat wizard things. Wizard/Fighter or Fighter/Wizard
Concept 2: inborn magic who then learns to master his magic. Sorcerer/Wizard.
Concept 3: wizard devoted to the god of magic. Wizard/cleric.
Concept 4: nature priest. Cleric/Druid.
Martial:
Concept 5: Young Conan a barbarian who becomes a thief. Barbarian/Rogue.
Concept 6: Ninja! Rogue/Monk
Concept 7: Warrior becomes the chosen one Fighter/Paladin
Concept 8: lightly armored wilderness warrior. Ranger/barbarian
Concept 9: Dirty tricks sneaky fighter Rogue/fighter
Pretty much any full caster has to multiclass very carefully to work well as a multiclass (concepts 1-4).
Pretty much any martial class multiclasses fairly well with others (5-9).
Take an unfocused scattershot martial character build, say start with barbarian and add on random compatible martial classes.
Barbarian 2, Rogue 2, Fighter 2, Ranger 2, Swashbuckler 2, Scout 2 and he's not bad at facing level appropriate combat challenges a 12th level party is expected to face. Throw in fractional BAB and saves and he's roughly comparable to any other level 12 martial characters.
12 levels of any spellcasting multiclass has to be carefully handled to take on the same type of combat foes.
These are not wierd corner cases but a breakdown of the design goal when applied to multiclass spellcasters.
There are options to come closer to the design goal.