For logos, it's not dangerous at all.

For kernels, as long as the kernel was carefully written, there's little danger of anything going wrong.

The worst thing that can happen is that your player will stop working properly because of a bug or because of an incorrect kernel file. But at that point, all you have to do is re-apply one of the factory upgrade files. This will replace all of the kernel and player software, putting it back to original factory specification (without even hurting the music).

Even if the kernel completely trashes your hard disk (highly unlikely, the person writing the kernel would have to be either deliberately malicious or incredibly careless for this to happen), you can always rebuild the hard disk from scratch using the disk builder software, apply an upgrade, then re-load all your music.