I think focusing on the size of the item and the plot impact of "People will WANT this" are healthier than that hard cap.
In addition to these, and just letting the GM say no to something that breaks the game, I'd also let the GM force the player to go through the full creation process of the item if it takes things too far.
Player: Alright, I use my 28 enchanted item slots to make a weapon: 30 that can be used 5 times a session
GM: Sounds cool, let me make some quick notes. Does a base complexity of 60 sound about right for that?
Player: Ummm, what?
GM: You know, the complexity of the enchanting process to make that monster
Player: Well, I was just planning on, you know having it, and...
GM: And which house are you using?
Player: Excuse me?
GM: Well, where are you getting the house that will be turned into your enchanted item?
Player: This is starting to sound like too much trouble
GM: Of course, it would be a real shame if the local bad guys shot up that house after you enchanted it. It might degrade the items effectiveness
Player: Look, can we just forget this?
GM: Aww man, now I have to design a new plot that doesn't revolve around everyone trying to destroy/control your death star.