with people who use PLCs on a daily basis
I have been working as a service engineer with siemens PLC's for 5 years or so, upgrades, updates, bugs, etc. As one customer explained, there are no problems only solutions. It also depends on who you will be interviewing. In short you have 3 or 4 main groups. Maintenance, software engineers, service engineers, and maybe remote support.
The questions will vary greatly. I have been dealing with so many customers, and I hear more often than not that they will prefer ladder logic. And why is the program so complicated. They always have a thousand and one ideas on what the program "should do". Also customers hate STL for some reason.
For me as a service engineer. Don't name a variable Motor_Fault and have the bit be TRUE when is normal. Variables naming should do what is called. The company I work for is big, and I work with software engineers from all over, some use STL, others FBD, SCL, a bit of ladder. I enjoy all except FDB the scrolling down is insane. Other than that timestamps is another big pet peeve that even some senior engineers do not take seriously.
I don't have any issues with siemens PLC's you adapt to the tools you use which is simatic manager and tia portal.
In short the biggest problems are there are wayyyy to many people hooking up to PLC's who shouldn't even own an ethernet cable or mpi adapter. Over-engineering of programs, what makes sense for you as a programmer might not make sense for maintenance personnel. If you have a piece of code that is complicated, at least try to comment your code well.
For customers issues I think so many come from the good ole' ladder logic days, that is hard for them to comprehend FB multiple instantiation and why that is good in terms of maintaining code. And when downtime is crucial maintenance personnel should at least get acquainted with the program structure for example which one is the Main function call? Where do all the motors get controlled from? So basically breakdown the program and have a cheat sheet, preferably do all this before the machine breaks. Learning while a machine is down, is not the best way to open the program for the first time.