You might be right, but by theory it should still be a playable char, eventually a bit weaker but still able to be run with single class of the same total level. Or do i read to much into 5e s principles?No you are not.
8d6 fireball is very good at 5th level but pretty bad at 10th level. At 10th level it's more of a mook mop-up spell.
Same with extra attack.
Not to mention that you still can't do 2 things at once(except one instance of Action surge). You can either attack or cast a spell.
at 11th level(start of tier 3) every 6/5 split is worse than any single class of 11th level(well, maybe not ranger or monk ).
you can maybe make some cheesy 3/8 or 2/9 dip that could go above 11th single class power level, but this suggestion is also to prevent 1-3 level dips.