If you have ever flown on a minecraft flying machine without a boat/minecart it is really hard to stay on it. Then, add in the people amazed that someone is flying, and everybody starts shooting at them. With all of this in mind, I don't really think it's op.I think it is too op. Skywar will full of flying machines.
I think this kit will only give you 3 slime blocks and 3 redstone blocks, so players have to make pistons themselves.