That is true! Feedback given by the job is more effective than formal appraisal from your boss, because their are no surprises about your rating at your job and everything is planned out; on estimated evaluation & rating.
Difference between informal & formal appraisal
Informal appraisal- your boss evaluates, open feedback while getting coffee, or during work. (Early feedback is more effective then surprised work results)
advantage- knowing your weakness and improving on job, beforehand evaluation, efficiency
Formal appraisal- surprised work evaluation of the job- disadvantage- not knowing your results, weakness on job, inefficiency.
Hope this helps :)