Yeah, my current method seems to be fine with several instances on screen at once, so no problem.
This morning I did something similar to what you mention with a horizontal moving platform.
For some reason, adding both a Physics behaviour and a sine behaviour to the same object causes the sine behaviour to misbehave.
With just the sine behaviour controlling the horizontal movement of the platform on it's own, it moves correctly.
But with the physics behaviour added too, the sine behaviour causes the platform to not reach it's offset each turn, which means that it eventually edges it's way off screen and doesn't come back.
I solved it by using the dummy with the sine behaviour for position, and the platform for the physics behaviour.