* Rough terrain
* Adverse weather conditions
* Dealing with civilians
* Going up and down flights of stairs
* Taking prisoners
* Medical care
* Being underground
probably a lot more
3CronoDAS
I'd guess that communications are a problem - you'd need more bandwidth to send enough video back to drive a car remotely than to fly a plane, and it's probably easier to lose contact, too. Not to mention the difficulties of fighting inside a city you don't want to simply destroy: can your robot open a door and go up a flight of stairs?
This is the kind of thing that's being researched by the dreaded Military-Industrial Complex, though.
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