I bought Ninjas recently and also some carbon balls for my games with magnets. I thought I'd use the Ninjas in my GnR because it only has a magnet in the upper lanes which really isn't activated all that often - say compared to The Walking Dead for example. After maybe just a few games on my GnR with the Ninjas I noticed some trough issues and immediately realised the balls must already have some magnetising issues. I swapped for the carbon and then trough issues all gone.
The Ninjas are fantastic in my opinion but definitely not suitable for any games with any magnets at all.