Frame grounding, data & power - EMI
I would highly recommend that you read ETG1600: Guideline for Planning, Assembling and Commissioning
of EtherCAT Networks , a public white sheet that covers the suggestions for system design and covers grounding considerations.
In EtherCAT the signal processing chain is digitized and goes through an EtherCAT slave controller or equivalent on every single device.


EtherCAT Network Topology courtesy of Beckhoff

Therefore, The Daisy chain topology of EtherCAT is not actually a physical daisy chain and does not resemble a traditional bus interface of E.G RS485 or CAN.


Signal processing chain inside the slave device, note that up to 4 ports are allowed allowing for a star topology, but typically 2 are seen on devices, the ethernet frame processing occurs in the &quot;EtherCAT processing unit&quot; between the input port and any subsequent network segments.
Courtesy of Beckhoff

Furthermore, EtherCAT relies on specific physical layer requirements for isolation and grounding. 100 Mbit Ethernet over CAT5/CAT5e should be isolated to 1KV via transformer. As long as your frame voltage does not exceed 1KV between two nodes, you should not have any impact from frame coupling the devices.
In fact, the typical recommended scheme specifically recommends against connecting frame shield to ethernet shield in either case. Although this will not necessarily be enforced by spec.


Ethernet Grounding recomendations for EtherCAT slave device developers
Beckhoff ET1100 Datasheet


EtherCAT does not address power grounding requirements, this is left to sector/industry/product specific requirements. Furthermore, EtherCAT does not specify a novel physical layer, but relies on existing specifications for physical layer (e.g. fiber, gigabit, ethernet)
Ultimately it is up to the system designer and in some cases an industry consortium to decide what is the grounding scheme for their systems. For example
If isolation and/or long distances (1KM-20KM) are required - a fiber physical layer is completely isolated.
Ultimately, if the typical isolation grounding scheme of ethernet devices, in particular the tendency to couple frame ground to ethernet shield, in case of shielded cables, may present a challenge if you are also using frame-coupled DC distribution systems.
Conflict of Interest Disclosure: I have some affiliation with ETG