Second one does look underexposed as the sky looked like it was metered instead of the subject. I usually shoot in Spot Metering mode and Single point AF and hardly ever get underexposed subjects, especially on a beautiful day like you had.
First one is a classic example of shooting in bright sunlight during portraits. Her face, which imo is the most important part, is hidden in the shadows. Simple positioning change or fill flash would have fixed this on the spot. Pop up flash is brilliant for fill flash, but not much else haha