I was invited to the Assessment centre in Milan which was very poorly organized. It started with a 45-minute delay (no apology for that from the HRs), and, after a morning dedicated to a group exercise, it transformed into an afternoon spent waiting to be called in for interviews. There was no order, program, or schedule. Some attendees had 2, others 4 interviews. Nobody knew when they were going to be called in; I, personally, had to spend 2 hours staring in one point waiting for something to happen.
What irritated me the most however, was the fact that those of us who were not accepted did not even receive a rejection letter. No rejection, no feedback (even though it was promised!), nothing. I feel like just writing a brief email to the candidate to inform them politely that your decision is negative and to explain why should be the norm after attending an assessment centre. This is not a mere interview; we dedicated all our energy to the AC, only to be ignored.