The Mii Fighters were hurt because they didn't even try to walk the line between generic and being actually fun to watch. Their movements have no flavour and their special attacks are mildly interesting at best. The ARMS cast would wouldn't feel pressured to be an every-man so their personalities would have a chance to shine. From a balance standpoint the characters are likely to have an ARMS switching mechanic, so their attacks should already be balanced around the ability to change properties. After that, any deviance from Spring Man would be balanced against him instead of the rest of the cast, making it much easier since they can balance the characters by strengths and weaknesses over Spring Man. Now, I don't think every character would work this way; Dr. Coyle's abilities are very obviously good whereas Master Mummies obviously not, and Helix's abilities don't even work in 2D. However, If we're just working with Spring Man, Ribbon Girl, Ninjara, Min Min, or heck, even Twintelle and Kid Kobra, I think they'll be fine.