Does anyone know if there are known bugs with the plugin?
No, I have been using it successfully since dropping my raid 5.
The SD cards are connected via two different USB-ports on my Rock64.
I am wondering if this is the problem, you say you are using SD cards via USB, are these SD cards in USB card readers?
There is a user connecting 5 x 2.5 drives via a USB charging hub connected to a Rasp Pi using MergerFS + SnapRaid successfully.