I've never had to walk any of the cats I've owned.
Cats take way less upkeep. The fur being everywhere is a downside though. Oh, and nice and relaxing toilet sessions are impossible...
Even if cats don't need 'to walk' they have innate instincts to hunt that need to be fulfilled, it can be tricky in urban area. You need extra accessory, DIY or commercial, to help with that, and your own time. Spending time 'to play' with them, or simply spending time with them, is too necessary otherwise say goodbye to your sleep schedule or schedule in general, and to prevent potential health issues. Since they are nocturnal your need to 'overwrite' their instincts 'manually' to suite your need (to sleep). Depending on cat they can get various level attachment to owner and demand attention at random moments, and getting irritated if don't get it right away, prompting them to behave badly (similar to children) just to get your attention. This brings us to next problem.
The Claws. You need not only Scratching Post and board but a few of them. On top of that you need 'to train' them to use them or say your furniture goodbye. The biting instinct can't be overlooked, each time i go in bathroom i fear for notebook charger and others cables in general, already lost few headphones. Its get worse if don't give them attention. And to deal with fur you need to comb them regularly twice a day, depending on the breed can be a nightmare.
In general its true that having cats is much easier than dogs but after substantial time investment. Like i said it depends, on few factors.