Reason for instability possibly because Null cannot be compared to anything else because there is nothing to compare. However, expression of Null >=0 should be treated as a False result because it certainly can't return True.
Never build a comparison expression with equivalent operators when Null could be an input. Use IsNull(), Nz() or in SQL IS NULL.
Note suggestions using qualifier prefix of Me. Can wrap in With Me structure.
Code:
With Me
.ControlName.Enabled = NOT IsNull(.OtherControlName)
...
End With
Could use With anyobjectreference to condense code.