We have a few jmks in our shard. I've only fought one of them since the others are usually still low enough to go around and I usually have to refresh once on my climb anyway so I usually play it safe and take the easy wins since I have excess battles anyway.
The one at the top (the one with the best mods in my shard) is beatable but not easy with see. I probably win about 40% of the time. There's also a Rey team that has a payout between mine and his so usually I'm already in first but occasionally in 2nd. So it looks like Rey can also win fairly reliably too.
This is far from broken. Though I'm not a huge fan of the permanent taunt Gk gets. I think the team would still be good enough without that unintended interaction.
I expect the new GL to perform like this. If he was an easy target in arena, then there would be little point in getting him. As someone else mentioned, power creep is part of the business model. It always has been.
I'm no whale but I am working towards jmk since I know that the see and jml will not be arena viable forever. Based on slkr and Rey being even older characters, they will also likely not be arena viable either. I skipped them altogether since I don't have the resources to gear everyone and at this point I probably won't ever go back and get them.
Part of that is dislike of the sequels but it's also the reality of the game that you may have to skip things if you're ftp.