It actually can cause harm if safety guidelines are not followed.
The iPad may become excessively warm during use.(Especially iPad 3 or 4 with screen brightness turned way up) Skin contact to the warm area for extended period of time may cause low-temperature burns, it's kind like boiling a frog with warm water, when you actually find out that you are burned, the damage is already done.
So do as the user's manual said, avoid touching warm areas of the iPad.
It has nothing to do with the RF, though. Cellular or Wi-Fi/Bluetooth RF signals do not cause any harm, any claims for harm are BS.
iPads pass all FCC regulations and there are extra measures to prevent excessive RF absorption. All iPads have proximity sensors inside the cellular antenna so when you touch the antenna window, they automatically reduce transmit power. This is the reason why Apple can make the antenna a hand-holding area with your hand literally all over it and still passes all RF regulations.
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