While I know it's not good enough reason to prevent Halberd being neutral alone, I think that it being such an exceptional stage for Olimar (camp under the platform whenever the slopes are present=WIN!), combining it with the ridiculous advantage it gives multijump characters (imma shark!) prevents it from really being true neutral.
I do agree with Forward. None of the stage hazards are particularly... hazardous. You see the claw and bomb from a mile away. If you get thrown into the beam, who cares? Super DI out. It's not even hard.
Still, I think that the neutrals should be Smashville, Yoshi's Island, Battlefield, and FD, but that's just me, and most of CO.