I honestly don't understand why everyone thinks this feature will be broken.
The most straightforward approach to balancing out the Miis would have been to give them all standard height, weight, and hitboxes. I know this goes against Mii customization and is a lazy solution, but it's also a solution that would work.
Trying to "balance" this feature by giving various hitboxes different speeds and weights is just another lazy man's way of attempting to balance things. Like you said, having different sizes already changes the hitboxes by some number of variables. The problem is that changing a character's speed and height doesn't balance this out, it just adds more variables. It's like how characters have different amounts of health in the vs Capcom series. It kinda works to an extent, but in the end it doesn't really balance things out all that well...
We're also talking about Sakurai here. He's pretty piss poor when it comes to balancing things. It's not so much that I think this system can't work, I just don't believe that Sakurai cares to balance it. I also don't think he could balance it if he tried.
Exactly my reasoning. If competetive players don't want items or certain stages for balance issues that's there prerogative, but in a non-competitive setting I like to have my items on high and my stages varied. No reason to go about bashing a feature you don't like if you don't plan to use it but maybe 90% of everyone else does.
And yeah, but that's not really what we're talking about here. I'm more of a casual player than I am a competitive player by a long shot when it comes to Smash, so I enjoy things like items, customization, and unique stages. The majority of my Smash play sessions are with friends and we usually aren't too serious.
The problem with the Miis are that these features begin to
unnecessarily exclude things from competitive play. It's just a poor design decision considering it could easily be made to appeal to competitive players as well. Items and stage hazards are one thing. They're fun, and they can also be avoided for people that want to play competitively. But having these variables within a character means that they're really only useful for casual play, which means that they spent an awfully large amount of time and resources on characters that only appeal to some of the base players. They could have just as easily made Miis more unified, meaning less variables to worry about with regards to balance. This would make Miis fun for everyone, while still making them interesting and viable for people who like to play competitively. I'm sure the Miis will be fun, but it's a shame that Sakurai makes such poor design decisions regarding things like this.
Now, it would be nice if there was also a default set of standards for the Miis. In other words, a default load out with preset height, weight, and special moves. This wouldn't take anything away from the fun of customizing and experimenting with the Mii fighters, and could also provide the competitive players with something that they might feel comfortable adopting into serious play. Alternatively, the competitive scene could attempt to adopt customization for a while to see if it's viable for competitive play. If it's not, then they could attempt to create standards for competitive use. Or just ban characters left and right. It doesn't really matter at the end of the day cuz the game is gonna be hella fun no matter what, I just wish Sakurai wasn't making things so difficult by adding in such unnecessary levels of detail.