What are the advantages/disadvantages apart from the obvious loss of one gear in the middle somewhere if both drivetrains start and end on the same size cogs
You get an extra gear ratio and closer spacing of ratios with 11 speeds. 10 speeds drivetrains weigh 1 sprocket less I guess, and 10 speeds chains and cassettes are cheaper too.
1×11 has extra gearing range, that's good. But you have to know, a 11 speed chain is thinner than 10 speed, so you should pay more attention to gear tuning. Spare parts for 1×11 are less common and more expensive. I don't know exactly, but I suspect that 11 speed transmission will be less reliable. In my bike store working experience, a SRAM GX group offers a rear derailleur that has a stronger spring; your chain will stay on the sprocket in most situations, but shifting would require more force.