Is it safe to run long I/O wires to a microcontroller?
I have had personal experience with network cables melting during a lightning storm. In almost all cases of failure due to lightning it is because of bad power or ground. If you end up having 2 devices far apart from each other and each has their own ground, it is possible for the ground potential of one to change because of the lightning and then the small gauge wire in the Ethernet line ends up taking on a lot of current which results in the insulation melting.

The same thing can happen if you have a surge on the power of one device; it can cause the communication wire to pass a lot of current down it.

If your "external" device has its own ground then you might need to worry a bit, but it probably won’t be that big of deal (especially at 10m).

You will need to worry about noise. The longer your run is the greater chance you have of picking up noise. This may not be an issue for what ever it is that you are doing, but you do need to keep it in mind.