I thought it complicated the estop circuit, so now your not just verifying hardwired devices for correct functions, all that wiring that was in a physical panel is now in the estop programming. So now estop is not just wiring but wiring, network setup, and then the programming.
If you only have a simple system for example consisting of a single E-Stop relay, a safety PLC does not simplify things. But if you have anything slightly complex, a safety PLC can simplify things considerably.
As a matter of fact, in order to reach the highest safety levels, a safety PLC can be the only way to achieve it.
Too much fun and sometimes easy to bypass if you dont know what your doing and sometimes easy to wipe out the programming.
It is harder for a maint without the software to bypass the safety. If you have RFID sensors instead of more simple switches, then it is very hard to bypass.
Even if you do have the software and the program and can change the safety program, then you cannot hide that you have made a change. That is what the 'signature' that James Mcquade mention is used for.
A safety PLC also makes it easier to test and document the system. This is getting more and more important. The days of an uneducated noob slapping a control system together, stick an E-stop relay in there somewhere, and then all is OK is over.
Here is a tip for James Mcquade:
Since you are going to work with safety, and since you are in the US where you can land yourself in jail if you get to be blamed for a serious accident, be sure to document everything.
Risk assesment, verification, validation, protocol everything.