ModMii has never once bricked a Wii.
ModMii is very different than pimp my wii, ModMii runs on Windows to get all the files u need to softmod your Wii properly (and without requiring wifi on your Wii) and builds a custom guide for you to follow. Basically, u use modmii to put all the files on your sd card and build a guide, then u put the card in the wii and follow the guide. So I would call it an interactive wii modding guide rather than a hack-o-matic tool like pimp my wii.
I personally don't trust hack-o-matic tools like pimp my wii or sciifii that run directly on the Wii and the end user has no idea what it is doing. I especially don't trust tools like that if they will go online to grab files from nintendo (just look at the "apps that brick" thread, almost all of them used to work at one point in time until ninty put out an update and the apps started downloading/installing stubbed files which lead to brick)
So would I say ModMii is safer than pimp wii wii? Well, ModMii has never bricked anyone, and pimp my wii has... so I would say the facts speak for themselves.