We use the forwarding service to have our mail forwarded to our son's house when we are gone for 10 weeks or so. We have never had any real issues, however we did find out the first time that our bank's mail won't forward. So, as the OP experienced, some checks were returned to the bank. We don't have any magazine subscriptions to worry about.
I really think you'll be wasting your time complaining to anyone. I suspect the good or bad service received is dependent on which town you're in and the particular post office management.