You could get bricked if you start out with a softmodded wii on 4.1 and just update through the normal channel. If you start with a regular wii, or use a safe update method (read the stickies in this section) it'll be fine.
Lots of IOS won't work on 4.2? Really? Can you name one?
I certainly can't.
It's true, though, that 4.2 installs some IOS'es that contain no code (stubs). That's why updating a softmodded wii could result in a brick. However, those stubs can be deleted or overwritten by the previous custom ones, so I doubt that there are IOS'es that simply refuse to run (keep note that games need IOS'es to work...if those wouldn't work, nintendo would have a serious problem).
Can't say anything about hardness of hacking. I know you've got to keep an eye out for the version of the firmware if you follow tutorials (e.g. you shouldn't install a custom wii theme for a 4.0 firmware), but other than that, I can't say there's much difference.
To my knowledge, there are no games made for the USB loader (considering nintendo never supported it, that's kinda obvious
). As such, there simply aren't any games made for the USB loader.
And "scrubbed games" usually means that a long string of zero's is missing at the end of the data layer of the disc (games pretty much never fill the entire disc).
Of course you shouldn't download games to begin with. But of course you already knew this.
