I think it's also the quality of the SD card. Or it could be that using a micro card adapter causes problems?
The class 10 card (Kingston) I used, was a micro card inside an adapter.
Tested with an older "original size" (Adata) class 4 card I had.
Rasplex booted up perfectly.
Got myself an original size, class 10, card (Verbatim) worked first time.