What's wrong with this guide?
What obcd says: things are still in development. If you use a guide that hasn't been updated in two years, it'll miss out on the newer developments.
Perhaps more importantly: we can't be held responsible for what your random guy posts on the internet. It's not that hard to write a guide that works for you (with 'you' being the author of the guide), but harder to write a guide that works for everyone in any situation.
The best example is the people who follow guides that install darkcorp because they heard that'll allow them to play backup DVD's from the disc channel. And while that may be true, those guides often forget to mention that this only works on wiis that are older than mid 2009 (the newer ones have a different drive that won't accept anything but retail discs). And they don't forget that they hack much more than is strictly needed. Or they don't mention that playing games from USB drive is not only much more convenient, but safer and more reliable as well.
And the reason? You guessed it...because those guides were written before those new drives became standard.
To answer your question: no, overwriting IOS55 won't brick your wii* (
linky to some info about it). But unless you replace it with a hacked version, it won't play backups from the disc channel. And I don't know if modmii has those (why should they? USB loaders don't need to mess with system-crucial internal IOS'es and the DVD users are a minority anyway).
EDIT: you can re-softmod your wii by using modmii, indeed. Just do it from the start and treat your wii as if there was no hacks on it. At the worst, you'll lose some time reinstalling the same things, but it's the best way to be safe (and perhaps update some things in the process).
*there are many IOS'es on your wii. As long as you don't mess with the one your system menu uses, things are fine...in theory. In practice, the games that rely on that specific IOS may or may not work.
