Samurais need to touch the enemy to retaliate so they are terrible. Ninja's do not retaliate at all so you can just walk up and kill them and on the last few maps in GE there wasn't a single forest to hide in so they got trashed as OF units move first. And Ronins are dead before they can get in range to hit anything. Good luck using VF units to kill anything.
And since the recent improvements to the hover tank, they are even better at auto'ing GvG maps.
Ronins aren't bad, actually. It's very hard to kill something before it gets in range when that unit you're trying to kill can attack from 8 tiles away and has the strongest defense in the game. Again, using them against artillery and claiming they're bad because of that is like saying turtles are bad because they can't hit gliders. It's not the game's fault if you're trying to use units against stuff that's strong against them. Ronins usually take 1, sometimes 2 damage from fast and ranged units, even when I was using them in level 4 battles, and can easily knock them out in two attacks. Now, I won't argue that they're better than hovers but in situations where a hover's stealth isn't that helpful and you still want to use heavy units, ronins are very good.
Another helpful tip for anyone reading: If you're in VF and using hover tanks in GE, try slipping a VF unit into your army when you can. It'll alter the battlefield significantly, removing a lot of the forests/bushes and making it much easier for your hovers to move around while still keeping stealth.