PeekabuPi:
If you created some application with the arduino boards or using AVR or PIC chips, how good is their long term reliability and robustness?
Excellent.
PeekabuPi:
For example, PLCs are engineered specially to take a beating, long continuous cycle times, electrical circuit protection, and good mechanical protection against shock etc. So ultimately PLCs tend to be workhorses and great for automation applications because they are reliable for long term continuous use and are very robust.
Read these forums for a while. Arduinos take an awful lot of abuse. Remember: the connectors on the Arduino board go directly to the chip, there's no protection circuitry.
PeekabuPi:
Are arduino boards and AVR chips as good in terms of long term continuous operation? How reliable are they?
AVR chips are designed for industrial operation. They have built-in watchdogs, brown-out detection, everything you need.
The Arduino boards could be improved, but not terrible. They're designed for hobbyist price levels. There's more robust versions out there if you look.
The Arduino IDE/software doesn't enable stuff like brownout detection, watchdog. You can do it but not many people will. In spite of that the programs seem to run for months/years without problems so long as the power supply is good.
twice:
Honestly, the 8 bit AVR controllers seem to be used mainly by hobbyists. I have looked for a list of professional devices that use AVRs but I found no convincing evidence they have serious applications in practice.
That's complete rubbish. There's no way Atmel would survive if that was true.
They also wouldn't be making all those six-pin, surface mount variations of the AVR chips, etc. Do "hobbyists" use those?