Erm...it's true that if a custom channel doesn't have the correct settings for the banner of the channel, then it may cause the system menu to not load (this is called a banner brick). If you take precautions, it is easy to fix (just use the same wad manager and the same wad, but select 'uninstall' instead of install).
Of course, the precaution is that you must be able to launch that wad manager first. In other words: you must have a way to not have to rely on the system menu. Having bootmii as boot2 is such a way, but not all wiis can install it that way. fortunately, priiloader works on every wii. Just install that and you should be fine.
Oh, but it's not like the chances of banner bricking by installing are exactly high. If you stick to wiiware of your region, chances are at least 90% that it won't brick.