Thanks for making this thread, I get really tired of people thinking of FTC as "FRC Lite"
It's really more like "FRC Mini" if a comparison must be made.
The biggest challenge in FTC (IMO) is in complexity. Think about the ratio of the size of an FRC motor or battery to an FRC robot. FTC robots are much smaller, but the motors are not much smaller. And by ratio, the battery is huge compared to the whole robot (Same goes for the NXT vs the cRIO, and the motor controllers). So now you have to fit all that stuff, and your drivetrain, frame, mechanisms, etc, into a much smaller space.
Oh, and FTC doesn't get a premade drivetrain handed to them every season
Which I think is actually kind of a detriment to FRC, because a lot of students don't even understand how a six-wheel drop-center really works. (But I guess that's a discussion for another thread)