When should an SLC be used versus a rule?
You do not use SLCs to the exclusion of rules or vice versa. In fact, without rules SLCs cannot alert you to anything.
SLCs cannot not generate alerts. An SLC may cause an SLC type event to be generated. That event may trigger a rule, which in turn may have an associated action that generates an alert.The question is: “Which do I need to solve my problem?”