Is Arduino stable enough to use it for a door lock?
I'm not sure if the platform is stable enough to use it for something like this.


Absolutely it is. As I describe in my post about an RFID door lock I use a Uno (and some additional parts) to unlock my door.

Since writing that I improved the software as described here so you can reprogram the valid list of cards "on the fly" (without uploading new code) as the valid cards are stored in EEPROM.



My system has been operating for over 5 years without any problems, except once when there was a brownout in the area and I had to power cycle the system (possibly the RFID reader failed and not the Arduino).

My system "fails shut" so it keeps the door locked in the event of a power failure. However I have alternate ways of leaving the building in the event of a power failure combined with a fire.

You are talking about a servo rather than an electric strike. That is likely to be the weak point, and not the Arduino as such. You would need to ensure it was powerful enough to do whatever-it-is you are planning to do, and that it had suitable drivers (eg. MOSFET or driver board).

My system gets used dozens of times each day. We never have a problem with it.