I had a difference of opinion with a member of my supergroup concerning the Invisible Woman. He argued that she plays like a weaker version of Cyclops with more support skills, and is further disappointing because she spends less time in stealth than other stealthy characters. These points may be true, in particular at high level content, but I'm having a lot of fun playing her anyway.
Which gets us back to the other point about stealth/invisibility or lack thereof. The bad news is that my colleague is technically accurate that Invisible Woman starts combat from stealth (she is automatically and permanently invisible any time she is out of combat) but once she is fighting she typically does ranged combat like other non-stealthy characters. She can escape into stealth by kiting for a few seconds, but other characters can get out at the click of a spammable button that's also part of their DPS rotation. But ironically the good news is that you never need to fight any trash if you don't feel like doing so. Mobs will emote because the AI doesn't know to turn that off just because because the mob can't see you, but you can easily, effortlessly and painlessly skip everything you don't feel like fighting. This is also true for characters who can teleport, and to a lesser extent to any hero willing to repeatedly dodge-roll for their life across the entire map, but this is a ton more work.
Maybe the appeal for me is that she's the ideal character if you're too lazy to work at avoiding and/or targeting the enemy?
Getting back to the bigger picture, every game now has way more character variety than you can feasibly balance. Whether it's dozens of specializations for a smaller number of classes as in WoW or maybe only one or two builds for a really large number of playable characters in Marvel Heroes, it's inevitable that some will be better than others. It's probably okay for some of the less optimal choices to be fun despite this, though I guess that can be a problem in a game where a significant amount of progress is tracked separately for each character (i.e. you can't really use Invisible Woman to farm gear for Wolverine), or if you felt really strongly about a particular character due to the IP. Then again, sometimes having the game be relatively easy on the baseline leaves room for a less optimal character build to still get by with enough synergies and bonuses. (Another example is Emma Frost's diamond combat build, which is widely panned and sounded bad on paper, but I liked it better than her popular mental DPS build)
At the end of the day, it is not in the developer's interest for any of the characters to be bad, as they're making money selling all of them. Novelty characters are possible, but probably limited (and I don't know that anyone would have picked Sue Storm for this). Maybe this particular character is under-tuned (in fairness, her DPS does seem slightly low), but should the fun factor count for something?