I am a technical co-founder, and that means I build things. In a conventional co-founding relationship, I am be expected to give my business-oriented partner the final word with respect to strategy or direction, and he defer to me on how to build the product.
At the outset, this puts all the power in technology. Nothing is more important than getting the right product built and shipping on schedule, execution is everything.
Unfortunately, at any significant scale it's all strategy. Technology increasingly becomes a cost centre; either it works or it doesn't, either it ships or it sinks. Strategic decisions begin to lead technology choices, and the CTO drifts away from the locus of power. Even if she's explicitly invited to participate in strategy, the buck stops somewhere she is not.
Influence, and with it ownership and personal investment, whipsaws from one founder to the other with neither realizing it explicitly. Ultimately, technical founders almost always drift away (as we've seen most recently at Buffer) when the company hits scale and they're fully vested. And that sucks - any new blood won't be as naturally invested in the product without further dilution, will lack history with the team, won't know where the skeletons are buried, etc.
I hypothesize that balanced founding teams - two business, two technical, sales & business, etc. would have more staying power in the long run. Cases where there is no default division of labour and influence. I believe I've seen this pattern, but without a neutral data set I can't trust my biases. YCombinator has enough data now to figure it out... (\nudge nudge\)