I think the good thing is that Frankie is a man of many talents. He is a fine actor - when he wants to, and is given a chance to - he's a reasonably good golfer, racing driver and drummer.
In the past, he's said things like “Right now my focus kind of changes every six months, cause there’s so many things I want to do”, but now that he's pushing 30, he does seem to lack a bit of focus to me.
He's quit two bands in just a few years, and his recent movie and TV appearances have been either B-movies, which appear to stretch the limit of routine teen action flicks and comedies beyond the age boundary, or brief cameos of the type where someone exclaims "Hey, it's Frankie Muniz!", and he's supposed to look embarrassed.
I think if you're his age, it would be a good idea to really concentrate on one thing now, and pursue other interests just as a hobby. Otherwise, you run the risk of acting that's conflicting with music or sports, or vice versa, and you may end up with not much of anything, apart from a lot of money in the bank of course, but just that can't be too gratifying if you've got still so many years ahead of you.
Rich
P.S. As Shaun has remarked above, I do think that health issues play a part too, and I'm certainly not belittling them. He's had two mini-strokes now, and ever since he smashed his hand in a racing car accident in 2009, his hand appears to be playing him up now and then. I think all of this can make long-term commitments difficult for him, and can of course lead to studios and other companies passing him by, because they don't want to take any chances.