Street fighting ("combat in built-up areas") is the pits. It always comes down to which side has the best infantry and the most determination. It's nasty, ugly, dirty, grind-it-out combat, where you are generally better off fighting from the top floors down rather than from the bottom floors up, and where grenades almost always rule the day. (Toss a grenade or two into the next room/whatever, wait for them to go off, and then charge in.)
I can't think of anything much worse, except maybe tunnel/fortification operations.
I'm imagining clearing block after block of tight streets with nooks and crannies where Islamist scumbags are laying in wait.
Yet we appear to be kicking their asses, as they are clear to operate unhindered tactic wise.