It really depends on several things:
* What you outstations consist of, i.e. are there complex control requirements
* Reliability of communications infrastructure
* bandwidth available from communications infrastructure
Typically I use RTUs for standard, simple applications e.g. 2 pump wastewater pump stations, water reservoir, flowmeter station. For a treatment plant, almost always a PLC. Depending on communications infrastructure available i may still have an RTU on site just to bring back data to the master station.
The reasons I like RTUs for difficult or semi-reliable links is that their communications protocols are simple and bandwidth efficient and i can log compliance data locally in the RTU and pull back to the master when the radio channel isn't busy.
Conversely, RTUs are typically not suitable for complex control.yeah they can probably do most stuff at a pinch, the feature set is not always there, e.g. PID loops, IO scanning, online edits. I'm not familiar with SCADAPack though. I know a contractor who hates PLCs and throws RTUs at everything. His clients are always having issues and don't get good value for money.
I don't think you should limit yourself to one technology. You need to start by understanding what sites you have and their control and IO requirements. Then what features do your operators need on site? If they need local HMIs, will that work with your RTUs? Etc.
Then how your communications network needs to look to reliably bring data back to your base station. Then i like to consider what happens if you lose Comms. Can your pump stations still run without knowing a level in a tank? Have you implemented an interlock that will cause a sewer station to overflow if there's no permissive signal from a downstream station? Those sorts of issues need to be addressed as well.
I've only ever had the pleasure of implementing a system from scratch once. We went with a mix of RTUs and PLCs as we had wide geographical area and needed to use 3 different communications methods! PLCs at the larger plants and radio and cellular RTUs for the rest.
I've seen others with just RTUs... and there are definitely problems. Can sometimes be the wrong tool for the job.
I look after one 3 station AB PLC system that uses the DF1 radio option which has had no issues in nearly 10 years despite next to no maintenance. But i don't know how well that scales to say,100 sites.
Hope that waffle is of some assistance!
