@Corexy The majority of replies on this thread are from myself, @Phaedrux and @dc42, and we're all 'official' Duet3D support. We've discussed your issue off-list too, to try and resolve what might be causing this, but are equally baffled. I did say back on the 25th Jan that it might be time to look for faults on the boards.
For the unresponsive v1.01 board, it's possibly a processor failure (possibly due to over voltage or shorting), but could be a shorted/broken USB port, stuck/shorted reset switch, along with erased firmware (DIAG light on). With everything disconnected, and SD card removed, plug in USB solely and hold your finger on the main processor. Does it get hot? You should know after 30 seconds if it does. If not, inspect the board for shorts, particularly around the reset switch and on the legs of the main processor. @dc42 suggested looking at the 'spills' (yes, solder joints) on the back of the SD card. Post good quality, hi-res images of the back and front of the board; forum users are particularly keen-eyed! I'd probably consider replacing the USB port if the processor doesn't get hot, and/or reflow joints around the SD card holder and WiFi module, but that may make no difference.
On the somewhat working v1.0 board, it maybe a floating pin (poorly soldered/dry joint) or faulty/failing component, but could also be a USB socket issue. Does that processor heat up? Most of the above could apply to that, too.
Are these boards more unreliable than the current v1.04 boards? Probably not. There is more protection built into current boards (fuses and a few minor component changes), but nothing major; see https://duet3d.dozuki.com/Wiki/Hardware_Overview#Section_PCB_revision_history for changes. I still run one of the prototype Duet WiFi v0.10 with white PCB on my main printer! There has been the odd manufacturing hiccup, but mainly that's failing voltage regulators or poor hand soldering of through-hole components, and gets picked up pretty quickly.
Could your environment be responsible for failures? Possibly. I think you said in a previous thread about lighting strikes, and your use of surge protectors; it's possible voltage spikes are getting through and killing the board. Dirt and/or metal particles contaminating the board can cause shorts. I've seen dead insects cause shorts, too, though usually on higher voltage! Short circuits through poor mounting are a fairly common failure as well.
If you're not in a position to test the boards further, or make repairs, maybe someone in the community can help. See https://forum.duet3d.com/topic/13875/community-repairs where a number of forum members have offered to repair boards, depending on your location.
Hope that helps.