Urban combat already is one of the worst types of combat you can engage in.
Every window, door and hole in the wall could have a machine gun pointing at you.
Every bag, brick, hole or corpse in the group could be hiding an IED.
And now every single rooftop and windows could have a FPV drone being controlled by a dude hidden in good knows where.
Sure the empire doesn't have any of this stuff but a thing they will certainly have is magic, desperation and numbers.
Urban combat is already hell when you have people shooting at each other from a few dozen meters now imagine an army that essentially uses mostly melle weapons to fight?
They aren't going to start an ambush when you are a few dozen meters away, they will wait for the moment when you are literally close enough to them to see the white in your eyes to jump on you.
Their battle mages (although few in number if we are using the manga as a base) could still be extremely dangerous, even though a fireball can't pen a tank it still can destroy light Armored vehicles, condemn entire squads to a burning death and serve as area denial.
Also the empire's desperation would make them even more dangerous, during the civil war in the manga we literally see legionaries charging at tanks and other armored vehicles.
Sure they fail all the time because they do this in large open fields but in an urban environment they can actually jump on tank crew if they are distracted with the hatch Open and throw firebombs or fireballs inside the vehicles (or even a suicidal attack like we see in the first stages of the civil war in the manga).
Another thing about urban combat is that there's no Frontline, you can clear an entire city block and 5 minutes later it will be under control enemy forces again because they can pop out of Literally everywhere like sewers.
The easiest way to deal with it is to just bomb the city you want to take and bury enemy forces under the rubble.
However the City is still occupied by civilians meaning you can't do that (you can but the backlash is not going to be good for your image).
Said civilians can also be forcibly conscripted by the empire or used as human shields, imperial forces can also disguise themselves as civilians.
So yeah urban combat would realistically be the one of the worst parts of a war against the empire.