As long as they don't sit in the passing lane, I don't see how a fleet of vehicles moving at a consistent speed and not driving erratically will have any more negative impact on traffic than a human driver. Like other's have mentioned, it might actually improve traffic as you don't have people speeding up to get close to a person and then quickly slowing down, causing "phantom" traffic jams.

Also, if the Waymos are following the laws, and that causes problems... then maybe those laws should be changed? Especially if most drivers already don't follow the laws.

"but I do think drone-based delivery will be rejected by consumers if it ever reaches significant scale in non-rural areas"

I'd wager it's the opposite: Rural areas would notice the one off noise of a drone flying overhead because humans notice novel sounds more. In an urban area you already have other significant sources of noise (traffic mainly) and if the drones are ubiquitous it might just be more of a background constant that people tune out.