What they say today about end-to-end encryption seems like it should work fine from a technical point of view. It is entirely possible the Google is very good about this, and when implemented, it might work perfectly as stated today.
But I'm not talking about incorrect or correct and I don't care about fairness in presuming whoever's intelligence either, because the thing I'm talking about is more important, which is risk.
Large companies taking on big tasks that you don't pay them for is undeniably risky for many reasons. One, they screw it up today. Two, they don't screw it up today but they change it tomorrow. We know this because many of these companies have done things like this before.