Sword of Spirit
Legend
The mightiest monster in the game, Asmodeus, has an attack bonus of +10. And that's pretty much what you'd expect from a level 20 monster, given the math they're using and bounded accuracy. An AC of 30+ means even Asmodeus can only hit you on a roll of 20. So I can confidently say that magic item stacking as it is currently written breaks the game.
If they do what they should and give monsters proficiency bonuses to attack, Asmodeus would be +13. It's interesting to note that many of the lower-mid range monsters are getting attack bonuses right now (using the currently arbitrary system) that are the same bonus they would get from a standard proficiency bonus. It is primarily the very low level monsters (and those inconsitently) and the high level monsters that differ by 2 or more points from what proficiency from what the proficiency bonus would grant.
I sincerely hope they switch them to all use the same proficiency bonus as characters. It works for both simulationist desires and better game balance.
/tangent