The Current Bread & Butter Setup?

They do seem somewhat robust, but there is always a chance. Not to sound like a doom goblin.
Whilst I have no empirical evidence, not having the resources nor $$$ to test, I feel (for lack of a better word) that Ethernet does provide some electrical isolation as opposed to direct GPIO connection.
Ethernet also frees you from being shackled to a single platform.

Tommy provides some very good information. Whilst to forum may appear to be a little biased based it would come down to a few points.
Tommy's mention of performance.
Reliability.
Wide range of options regarding hardware.
Many many more inputs than via GPIO.
The fantastic support from PCW, even when dealing with cloned items bought off AliExpress, or those building there own custom boards using dev boards and hashed together sundry parts.
Customised firmware, based on the available source, is no issue to have built. So far I haven't seen requests for new firmware features. Maybe this has happened I can't say for sure either way.
And finally, which I almost forgot, the knowledge of many of the forum members.

unknown wrote:

Customised firmware, based on the available source, is no issue to have built. So far I haven't seen requests for new firmware features. Maybe this has happened I can't say for sure either way.

Does this qualify:
forum.linuxcnc.org/30-cnc-machines/31792...rofit?start=20#85721

I was think along the lines of "I would like the mesa firmware to support this piece of hardware I have" which may or may not have been retrieved from a Soviet UFO warehouse.

That was very close to what you describe, i had some Serial Absolute Encoders that would cause me much grief to replace mechanically, so it was magnificent that PCW made the firmware i could use with those encoders.
BTW, that machine is still in daily use, and never gets powered off.

Ah I see, I was some what interrupted by The Hound whilst reading and responding, my apologies sir.

tommylight wrote:

In short:
RPI can do at most 20000 steps per second, if lucky and with a bit of fiddling (this is from what i read here, i stopped using RPI since version 3B)

The 3B wasn't very useful for LinuxCNC. The Pi4 worked a lot better (and can be used with Ethernet Mesa hardware). The Pi5 is better again. 

The reason I suggested the Pi5 in this context is that if anything goes wrong with the Pi you can just slot in an identical Pi with the _same_ SD card and be back to exactly where you were. 
If using PCs it can be difficult to find an exact replacement. Yes a _similar_ PC might work, and might run properly with the old SSD. But it might not.