I think it's a really interesting comparison. (I think we can ignore the loss of POI as being irrelevant in this case).

The lack of a stroke around the shapes make it difficult to tell the edges of the roads (try counting the number of white roads between bounding yellow roads). Where roads get tangled it's hard to know what's going on.

I personally find the colour palate a bit juvenile and less trustworthy. That might be because I've got used to the more demure extant colours. But I've noticed a trend with 'flat' colour palates being differentiated primarily by hue rather than, for example, brightness. I don't think that's natural for the human eye (I have no data to back that up).

And yes, Google maps is already part of the way to 'flat'. But it's kept the level of detail required to actually be useful. I'm aware that the LHS was a quick hack vs Google having spent a certain amount of time planning their layout.