Others have noted the problem of comparing scripted media (like comics, cartoons, myths-and-legends, etc.) with non-scripted games that involve cooperative efforts.
However, I offer in rebuttal: The game was designed, in part, to try to capture these characters while claiming to be cooperative.
That's one of the critical issues at hand. Because people are absolutely right, Gandalf isn't meant to be "balanced" against Sam. They serve completely different functions in the narrative, and Gandalf is simultaneously vastly more powerful and vastly more hobbled than I suspect Sam could ever conceive. Likewise for Merlin and Arthur.
The Batman/Superman comparison is a lot more useful though, and is part of why it gets brought up so much. Batman IS, to a meaningful degree, meant to be one of Superman's peers. That's an intentional part of the storytelling. Further, Batman has several "powers" that Superman doesn't. Political and business connections, fear factor (few villains fear Superman, despite knowing his vast strength), vast fortunes that can be funneled to "black projects" without people noticing (seriously, the Watchtower was a hidden line-item in Wayne Enterprises' aerospace division budget!), and (depending on canon) greater intelligence and superior observational skills (sure, Superman has x-ray vision, but he overlooks stuff Batman wouldn't.) It's also at least implied that Batman is more resistant to mind-control and possibly magic, whereas magic is one of Superman's only weaknesses. And, narratively, they fill very similar roles in the story, rather than being radically different like Merlin and Arthur or Sam and Gandalf.
There is a sense in which the authors intend for us to see Batman and Superman as "equals" to some degree. For example, it's pretty standard Justice League writing to have the two of them mutually respect one another for literally diametrically opposite reasons. Batman looks at Superman and thinks: "He has practically godlike power, and lost everything, his planet, his people. If he wanted, he could rule the world, and we could hardly stop him. Instead, he chooses to be upstanding, to never take the easy way out, to always show compassion. And he lives his daily life as a mild-mannered reporter." Superman looks at Batman and thinks: "He lost everything as a child, watched his parents die at eight years old. I had a family, parents who loved me. He had a tragedy. And instead of becoming a debauched layabout squandering his vast fortune, he chose to be the power that helps the powerless. To fight the things that crawl out of the darkness, but never stoop to their level."
So....yeah, I really do think there actually IS some teeth to the Superman/Batman comparison, particularly when we then move from scripted media to unscripted gaming that claims to be cooperative. The problem is, the designers want to have their cake and eat it too. They want to have Gandalf-and-Aragorn (or even Gandalf-and-Frodo), while at the same time saying or at least implying that everyone's on equal footing.
