This leads to the situation that discs are not available when the computer wants to access them.
Same issue would therefore apply for eSata
I would agree with this statement. If an external eSATA device isn´t available in an mdadm configuration the raid status would be set to degraded too similar to USB.
Am I safe to assume that it does not have the same restrictions as USB when it comes to RAID setups?
If you pay attention that the external eSATA box is accessible from OMV, I would confirm this.
eSATA is meanwhile outdated because it is slower than USB especially in a port multiplier (PM) configuration, where several disk devices are connected by one eSATA connector. The bandwidth of one Sata connection is shared by all connected devices.
Nevertheless I think it is more reliable then USB. In my setup I use a snapraid / unionfs pool out of three 8TB disks for my movie collection. The disks are located in an ICYCube 4-Bay case and connected via eSATA directly to one SATA connector with PM support of the ASROCK C2550D4i motherboard. All disks are detected in OMV as separate devices.
I take care that the external box is switched on when the NAS system is booted. I have no stability problems. The only drawback is that the disk transfer rate is slower when more than one disk is accessed at the same time. This should be avoided if possible. But in my unionfs configuration if only one disk is active the availaible disk transfer rate can saturate a standard gigabit Ethernet connection (almost) completely.
The better approach would have been a host bus adaptor in IT mode with a connection to an external DAS box. But the ASROCK board has only one PCIe slot and this slot is already used by a 4x USB3 card, which I use for external backup with several USB3 disk enclosures.