Those aren't weaknesses. No more than 'he isn't immortal,' or 'he can't regenerate' aren't 'weaknesses,' it just means that he doesn't have *every* power...
And she almost proves my point, being really powerful, and kind of dull.Captain Marvel is a much more powerful energy blaster. She can fly through space, superhumanly strong and fast, invulnerable, plus her cosmic energy powers and absorption.
Also not Scott's brother with an identical 'glass cannon' powerset, so less useful as a point of comparison, which was 'very similar characters.'Look at what even plain Jean Grey can do. Way more powerful than Havok, not counting when she's Phoenix.
The Summers dudes are almost a perfect example. Scott, weakest overall, most successful and popular of the three. Vulcan, awesomely powerful, lame as hell. The last Rasputin brother, Mikhail seems like another example. Vastly more powerful than his much more popular and successful siblings. That Braddock reality warper? Most powerful. Least storytime of the three. While being more powerful, or even cosmically powerful, does not guarantee that a character will be boring (or have to be written like a chump consistently to prevent them from overshadowing co-stars or annhilating plots), it does seem that there is a spot where it becomes harder to write meaningful stories for them (certainly not impossible, as decades of stories about Thor, the Silver Surfer or Dr. Strange have shown, but challenging, especially when they are supposed to interact regularly with things that are challenging to a team of people with a *single* mutant power that might not have the ability to do everything.