Do they understand the true cost of a highly available environment? Do they understand why feature development takes so long?
Even people with a limited technical background make those. In fact, they're probably worse, because they think they understand something but really don't.
Have two founders from different backgrounds, with a small understanding of the other is good. But the key is to understand their own knowledge boundaries and let the other take over where it matters. They need be able to play devils advocate to each other just enough to come up with the right solution.
I vote for "causing friction internally and hindering productivity."
I guess it's pretty silly to start a big project with people you don't trust though.