In your opinion, what are the bad consequences of neglecting city planning in the newly established cities?

Although I am not an urban planner or a city/town planner, I do have some ideas or understanding to share.

The general idea I believe is to arrange the development of type of structures so as to maximize the efficiency and with a look ahead at the future expansions.

What we might encounter if we didn't plan is a huge list. It also depends on the growth rate of the town/city.

The main failure will be development of motorways or highways. These are the main arteries of any town or city. the planning should start from there.

Amenities like Hospitals and schools get the priority as for their ease of access and importance, followed by Police and government offices. Commercial areas shall not be so far from residential areas that long commute is required for simple activities.

All these essential factors shall be within reasonable reach from residential areas. 

If you don't plan you can still work around may be most of the issues, but you always end up paying much more than that was required if you had planned. More resources spent and probably the development will be very slow than if properly planned. As because the budget required may be not within reach and have to wait. 

also if we go to the industrial side of the issue, pollution of air, groundwater and other resources shall be having a very adverse affect on the development of the town. rendering it with "not fit for living" category areas.

The appeal to bring in more residents can go down, in short; the town may grow, but happiness index will be negative. And the worst case is that it could also die down.

Non neglecting areas are Transport, Shelter, Security, Food & Water and climate

For a practical answer anyone of us can visit the old quarters of the cities in our respective countries. You can't imagine taking a car into the streets. If aesthetically astute, alignment of streets might hurt your aesthetic sense. Improperly planned drainage systems are another highlight. Several other relevant aspects are also mentioned by Sheryar. 

The bad consequences? Briefly listed some of them:

Changes of :

- climate, natural and water resources, biodiversity and green spaces, quality of urban air and noise levels;

Overloaded:

-management of urban waste,

-urban consumption;