SAP HCM and SAP ERP on single vs multiple instances

Hi All,

I'm doing R&D on HCM / EPR instances.

I wanted to know the advantages and disadvantages of having SAP ERP and HCM on a single instance/server vs different instances and vice versa.

Basically, I want to know what is the trend - do most companies have their ERP system and HCM system on the same instance or different instances.

Could someone please share with me there implementation experience.

Also, if you could share examples of companies who have implemented HCM or share case studies, I would greatly appreciate.