We have seen far too often pictures like these. It is disappointing and frustrating. Actually, no, the exposures on the film are perfect, as shown by the slides. The lab messed up. When you got such a bad machine print, you would not be confident that a properly exposed print could be any good, and thus hesitate to shell out more money to have a custom print made.
Slides are good for mastering the exposure.
From negatives, due to its color cast, it is not easy to judge by naked eyes whether a negative is properly exposed, unless the exposure is way off the scale. The wider tonal range adds another level of difficulty since, from the negative, you can still see most of the details. Slides are totally different. Any subtle difference is readily observable. From slides, now I see the subtle color rendition of different lenses.