Can an advising alert be submitted only if a student is doing poorly or struggling in a class?

No.  An advising alert can be used to provide any type of feedback to a student on his/her performance in a class. 

Advising alert includes a kudos function to support positive feedback for our students who are doing well in a course rather than just feedback to those students who are struggling.  Faculty have the option to send students “Kudos” to provide positive feedback to students who are passing courses when the Advising Alert is generated.