Independent of capped convertible notes being problematic under some assumptions (essentially, when the market turns down and you raise at less than the cap -- that's 90% of the problem, and 9% of the rest is when you don't then sell for enough to blow the pref out of consideration), an investor trying to lowball you on the cap is no different from lowballing on equity valuation.
If everyone is investing at $12mm, and an investor says "fine, but I'm special, give me $6mm or I won't invest", there's no difference if it's capped convertible note or equity (except that it's technically more difficult to do the investment as equity with different valuations; you can hack it by giving warrants or other benefits I think.)
Maybe it's not that the founders don't understand convertible notes. Maybe they are taking the funding at a lower valuation because they're willing to gamble on the likelihood of a (big money, not acqui-hire) acquisition by Google later down the road is more likely.
Google Ventures, in my experience, tells entrepreneurs that they have little to do with Google. They do not invest in things because they are synergistic with Google, and they may even invest in things that compete with Google. They make it very clear up front that Google Ventures is a separate organization. At most, they may tap into their network within Google to assist you if it makes sense, but any investor worth their weight in Silicon Valley can connect you with Googlers and other important companies.
