At my last job, our AS/400 had to have all of the applications shut down to do a nightly backup. The backup took nearly every second that the business was closed. Scheduled maintenance had to be done on holidays.
One time we had to move its network connection to another switch port. The thing didn't work again until we hard rebooted it.
The software on it could only be accessed from the network by running CL scripts - so there was no such thing as transactional integrity. The programmers used a five digit batch number on the main thing we used the AS/400 for, which recycled every month. As a result, if we had a few production glitches in a 31 day month, everything went to shit. The idiots also used a six digit invoice number and we rolled off old invoices after two years (which had regulatory implications). As we grew and processed more than 42000 invoices per month, hundreds of hours had to be put in to expand the system capabilities.
It's not all roses in the AS/400 world.