First, it's mechanical, so it's going to break down eventually. You'd better be handy with tools, because a call to the American Automobile Association ain't gonna do much good when your ion field rocket boots won't ignite and you're stranded at some supervillain's island volcano base.
Second, yes, it looks cool, but it weighs a ton and you've got to keep it somewhere. No matter how much room you make in your garage, it's not going to accommodate a Voltron-size robot suit. You're going to need an empty field and a large tarp, or maybe an abandoned airplane hanger. Ever notice how anyone who dresses up like a robot in comics is a billionaire?
Third, having a power that essentially requires strapping on or climbing into 10,000 pounds of nuclear ordinance doesn't exactly lend itself to leaping into immediate action. "What's this?! Doctor Deadly is attacking the orphanage! This looks like a case for the Incredible Metal Warrior! I'll be be back in a flash!" [pause] "...or, actually... with traffic on the expressway... let's say 40 minutes..." [pause] "I'll be back in an hour and a half to stop him!"
As cool as it'd be to have any super-power" well, nobody's getting laid with super-breath. We don't care how many orphanages you save using it, it just sounds stupid.
"What's your power?"
"I blow things!"
" Pfff! No, seriously, what's your power?"
If wasn't bad enough, it also makes you look like a dumbass while you're using it. Even Superman doesn't look cool all hunched over, cheeks puffed out, pushing air through pursed lips to knock out an arch-nemesis or blow out a raging fire. We're not denying that it's useful. It's just not terribly dignified. Plus, if you plan on using this power on a regular basis, you'd best have some breath mints on your person.
There was a 1960s comic book character by the name of Captain Marvel (one of the half-dozen or so who've used the same name). This is the version you've never heard of: a short-lived character who, upon shouting his magic word "SPLIT!" could make his body divide up into pieces that operated independently:
Yeah. Like that. It's as appalling as it looks, and thus would have the potentially useful effect of making your enemies violently wretch as they see you use your power. On the other hand, it'll most likely sicken the people you're trying to protect as well.
Like "super-breath," this just looks damned silly. Let's take the Atom, probably the most famous of all the shrinking superfellas. He can shrink to subatomic size, shoot along transmissions over phone lines, ride on top of birds... all pretty neat things to do, right? But at the end of the day, you're still a wee little man running around the ankles of friends and foes alike, and let's face it, you're just not going to look cool and get the ladies by being six inches tall. Well, unless you're willing to "go spelunking," but even then, we don't want to hear about it, pervert.