My children are adopted after the baby stage, so I haven't first-hand experience, but my experience as aunt and as godmother has taught me a lot. My sister had her children early, she was just 18 when the eldest was born in 1972. She and my brother-in-law coped well and did enjoy their babies, even although they didn't love the broken nights etc.
My god-children were born in the 1980s and 1990s to parents who wanted children, but had no first-hand experience of babies, never having had siblings or been baby sitters.
This I think is the important point. Today's new mothers and fathers are often only children. They just don't know what having children entails. A lot of grandmothers are still working, others live too far a way to be a help, and the young parents today, don't ask for help. So we feel we are interfering if we offer to do anything, or the young mother, especially if she is daughter-in-law feels we are criticising.
It's no wonder they are stressed. Some do resent the fact that babies take their mothers away from a satisfying job, others resent that the baby's daddy is even more handless and insecure around an infant that they are themselves.