Attributes don't work correctly.
The attributes should have been added together. For example when you have one item with +10% speed, and another item with +5% speed it should be +15% speed total. Also set -100 speed and -100 speed on mobs should have made them immobile.
Instead only the attribute of the second item equipped, or selected is applied. With the mobs they were completely unaffected.
1. Equip an item with the desired attribute
2. Equip, or select, another item with the same attribute
This will not add the two effects, instead it will result in the second items effect only.
1. Summon a mob with an attribute
The mob seem to be completely unaffected, at least for speed.