S5-115U was/is a very rugged PLC indeed.
One thing that fails repeatedly on S5-115U is the PSU. Notice how always the plastic on the PSU is less yellow tainted than the rest of the PLC. Its because they fail and get replaced regularly, every 5-10 years or so.
For current PLCs such as S7, I have experienced very few failures due to the environment. Some defects out-of-the-box, but very few once the system is up and running.