I like the concept of it, but I think its a little bit too abusable, since any melee player can bring these and it seems like it would cause a lot of trolling. Also, they would need to be really visable to the horsemen IMO.
You need 1 (maybe 2) points in Throwing.
They take up a slot (maybe 2) that could be filled with more throwing axes/xbow/whathaveyou that would arguably be more directly useful as a weapon.
So while any melee player could technically choose to put a point(s) into throwing and use a spare slot, most builds I know of do not have spare points or inventory slots for an item of dubious value. It is only useful as a weapon if people actually walk/ride over them. Otherwise they are just a deployable obstacle.
Yes, visually they should be obvious when close to them.