Drawback of Assignment look Up Rule ? for autopopulate AG based on cat and subcat

[Gk1503]

Hello Team,
I want to understand what possible drawback of Assignment could be look Up Rule.
If possible, please add any simple scenario to understand drawback of assignment look UP rule. 

[Elijah Aromola]

The only thing I can think of is that the task must be unassigned. It cannot have an assignment group/assigned to value or it will not run the rule.