They are not environmentally sound. Air transport is not as reliable as ground transportation. Bad wheather might ground the flight, or prevent it from landing at its intended destination. Airspace over most western countries is getting increasingly crowded. This means that it gets more common for every year that your flight misses its assigned time slot and has to wait for the next one.
Aircrafts have to queue for take-off, and are increasingly caught in holding patterns when landing.
Overbooking might lead to you not getting away until the next day or so. Syrebrist kan leda till irritation hos passagerare. It should be pretty clear, then, that if ground-based travel alternatives gets improved in regards to speed and comfort, this would be a real boon to air travel as well. If air travel loses passengers to ground transportation on some shorter routes, some airports would be less busy, there would be fewer flights, and it would be a let-up in this development towards waits for time slots and long holding patterns when landing. You can behold a long, illuminated ribbon of aircrafts awaiting their turn to land.