I much prefer single units to any form of double box although I think Michael Palmer's logic was that 2 nucs under the one roof help with heat in very cold conditions.
Using a Snelgrove board will get you a new queen but how many do that.
MBC. I think finding, handling and caging a queen is far more daunting for most beekeepers than mastering the grafting process. That is the big weakness in those systems where the queen has to be found and caged for a day.