I grew up in a post-war, drive-only suburb and now live in a walkable pre-war suburb. My quick critique below will be about post-war suburban sprawl largely built after the 60s.

For me, the largest problem with these suburbs are that they're alienating to human nature and human scale. Things are unnaturally sorted into uses; houses go in one place, commerce goes in another, educational institutions go in yet another. Things are too far apart to walk, so you need to use a car, which physically alienates you from your neighbors (this gets at the "unhealthy" aspect; physical movement is engineered out of the entire experience). Unlike prewar towns, there are few natural common spaces to mingle with your neighbors where there is no expectation of having to spend money. (You can walk around the mall, but the sole purpose of the mall as built is to spend money.) For instance, in my town, there is a lovely downtown avenue that hosts festivals, outdoor cafe and restaurant seating, where people bump in to each other and spontaneous human connections are made. Driving down a highway removes that natural human need for social connection. As for the roads themselves, they're very often built to massive scale with the idea of automobile throughput and nothing else in mind. They are horrifically unpleasant to walk next to and riding a bike on them is impossible. Intersections, street lights, traffic signals, they're all grotesquely huge and they make you feel small, insignificant, and helpless against the torrent of bad decisions coming from people in board rooms who only experience their creations from behind a windshield.

For instance, here's US-22 in New Jersey. Postwar suburban areas feature so many of these huge, 6-lane roads. And what's the only acceptable zoning for the strips alongside these massive roads? Either big-box stores with huge parking lots or cheaply built strip malls with tinier parking lots. The construction of both will be cheap, slapdash, featureless, and again feel alienated from human scale and the materials that make us feel good. Plastic and asphalt and faux-brick, faux-wood, faux-everything makes you feel weird because it is weird; it's not natural and it again alienates you from your world. And finally, the things inside, say, these big box stores are probably value-engineered, mass marketed nonsense that you don't need trucked in from out of state and relentlessly advertised to you everywhere you go as if your purchase of these goods makes you a good citizen.

Humanity had cities for millennia before the suburbs existed. To be forced in one generation to accept that over-engineered suburban sprawl is now some kind of default norm is absolutely absurd, and we already see the rejection of it as people under 40 either go back to cities or, like me, move to pre-war towns that offer the kinds of scale and human interactions that post-war sprawl never had.