The loss of flight for Kryptonians often doesn't matter much, given the Superman Ally team ability and tendency to have Hypersonic Speed. It has apllications, but the -1 damage seems to be the bigger draw.
The most effective use of Green Kryptonite I've ever seen was by a guy who had a rare Red Hulk carry it around. He didn't need to use it in attacks because he was using all of his actions positioning to Poison more people and stay out of line of ranged attackers, and giving multiple characters a damage penalty stretched out the length of Red Hulk's dial a lot. Because the Super Strength was from a trait and the object couldn't be taken out while he was holding it, there wasn't much I could do about it. And if I had been running a Kryptonian they would have been easier to Poison and had more trouble escaping. That was good thinking.
I thought you could target an object being held by a character?
I don't want to get into it since it is somewhat off-topic and would take awhile to state my entire case/opinion, but I would like to say that Planeswalkers are definitely NOT a good idea, as cards or the mediocre stories Wizards of the Coast crafts around them. The player as planeswalker original idea was enough, making them into characters did not work out (although I am willing to admit that with better storytelling it might have been okay).
Also, I need to make an effort to try and use the kryptonites more often. They are all very interesting and could add more fun/tactics to my games. Thanks for the article reminding me of that. As a criticism, I would like it if these articles could go into more possible uses of the objects in question, possibly including team builds that utilize them to their potential. For example, the blue kryptonite (I think it is called X-Kryptonite officially) works great with Stonewall carrying it, so he gets to hang out in hindering while enjoying his special power invulnerability AND is able to charge out of there without halving his movement twice.