Well, actually, it is internally consistent. There's nothing there that is inconsistent with itself. However, it's unbalanced as all heck, because it's twice as good as a wizard.
Not TWICE as good, surely? A L12 Wizard would get more bonus feats, HD, BAB, save bonuses, feats, free spells in his spellbook and skill points (off the top of my head) as a L6 SpellSage! The SpellSage is, of course, clearly superior - he has more spell slots available to him, and suffers no drawback to offset this (huge) advantage.
But you clearly grasp the point - it follows all the rules established elsewhere. But it is clearly unbalanced - IOW, other classes are clearly an inferior choice. If the Barbarian is the clearly superior choice to the Bard, then I would consider that a similar failure in game design. It's tougher to compare - the SpellSage example is clearly designed to highlight a lack of balance, but it simply places the choice of a Wizard as a suboptimal trap choice, rather than a viable alternative - but if the end result were that Bard will always be a suboptimal choice compared to Barbarian, then the rules need to be changed to make Bard and Barbarian equally desirable character choices. The goal should be an array of interesting choices, none of which enjoy clear superiority, or suffer from clear inferiority, compared to the other choices. Balancing all those moving parts is, of course, far easier said than done.
This was a large problem with 3e prestige classes and Wizards (or sorcerers), at least to me. Many provided the caster with the same spell progression as he would otherwise have, plus other special abilities, in exchange for a bonus feat every 5 levels (or spell swapping on occasion). Why would he NOT take up a prestige class - to remain a 20 level Wizard/Sorcerer was clearly sub-optimal.