I don’t think it would be an issue if it was implemented as weapons. For example, you equip a weapon called “Invisible Daggers” that is a mostly invisible dagger. It would also likely be made as a very rare/expensive weapon, like Volcanus, etc. so that everyone is not running around with invisible daggers, shields, etc.
The bigger issue would be when those weapons are brought to sPvP or WvW. For PvP balance the weapons would need to become highly visible at some point so that players can recognize and react to attacks (‘tells’) such as Kill Shot. So making the weapons have a visible form during skill activation would be a necessity; which would defeat the purpose of invisible weapons. Although there might be ways to use particle effects and transparencies to achieve an acceptable result; especially since the attack ‘tells’ are tied to the skills themselves and not weapons. So skills like Kill Shot still show their animation, just with a semi-transparent rifle.
