The Architecture guide  states: 
"It is still possible to have a configuration with a principal process server and additional process servers. Sage no longer recommends this configuration since release 2021 R1/V12.25. Sage recommends using the application cluster mode."

The Console help also echos the same recomendation "All the additionnal runtimes have the type MAIN (V12 only) (recommanded architecture)" [sic]

What does this mean in practise?
Sage recomend everyone to move to using Application Cluster architecture rather than using additional runtimes when you are able to do so.

To be fair, Application Cluster provides better performance as well as resiliance (no single point of failure) so is a good thing all round.

How do I move from additional process server to Application Cluster architecture
You can review my blog "Test system Build Diary: 2025 R1 (12.0.37) Convert a multiple runtime setup to Application Cluster (Multi-Main) runtime" to get an idea of the steps needed.

Conclusion
If you are doing a fresh install, migrating to new hardware, upgrading to new technology versions, or are moving from single runtime to multiple runtimes, you should be planning to use Application Cluster architecture, as this is best performing and most resiliant architecture, and is the only option Sage recomend.