1 - Soft PLC's can be much faster than Hardware PLC's (sometimes)
2 - Soft PLC's can save a tiny bit of money, especially if incorporated into an HMI station.
3 - Soft PLC's are extremely reliable in and of themselves (the software side)
4 - A Soft PLC's overall reliability and stability depends 99.9 % of the stability and reliability of the underlying computer/Operating system.
5 - Computer hardware and Operating Systems evolve and become obsolete at a frightening rate.
Because of 4 and 5, I strongly advise all my clients AGAINST soft-plc's. Even the Simatic type 'Sort of Soft' Card PLC's.
I can still purchase S5 CPU's, PLC-5's, SLC-500's, etc, that were designed into a system a decade or more ago. I cannot duplicate any computer that was purchased more than 6 months ago, if that.
A Hardware CPU is a controlled, stable, single-purpose enviornment. Anything running on a generic computer is not. Users WILL install additional software, they WILL apply upgrades and patches, they WILL eventually (even if ill-advised) connect the computer to a network.
Replacing a hardware CPU with a new one, and downloading the program can be accomplished in a matter of minutes. Configuring a new computer, loading software, configuring drivers, installing comms cards, service packs, and the like can easily take a day or more.
I feel very strongly, that there will always be a market for dedicated processing power. I'll stick with cards in racks