It will get very bad when they reach the actual city and civilians.
Indeed, the (minority majoraty) suburbs were abandoned when the IS took over for obvious reasons. As such the fighting has just been against IS picket lines. Even such token forces have still put up stiff resistance and at 1 point even surrounded the Iraqis though they've managed to break out and resume their advance. Once the screening forces get cleared out the real fight will begin.
It's worth noting that absent air power the IS fields a modern army, clearing Mosul will be like clearing Paris in terms of the kind of resistance we'll face. If the whole being welcomed as liberators thing turns out to be the BS it usually is we'll also be dealing with a multimillion strong fight with hostile civilians.