I've been doing a lot of backpacking in Chaco sandals. Chacos, of course, are not anything like Merrel Barefoots. But I think my experience is relevant.
At first I thought they were really hard. As in no cushioning. I would get tired after a while and didn't think I could keep up the mileage. Then there's the lack of a shoe to provide any support for your foot. My ankles would get pretty tired even though I haven't worn high-cut shoes in over a decade. And of course, there's the whole potential to stub or hurt your toes. But nothing has ever happened to my toes.
I've managed lately to hike upper 20-mile days wearing Chacos. I've hiked over snow and in freezing, snowy and rainy weather. I hiked a portion of the JMT in the summer and several 60-100 mile sections of the PCT.
I think sandals work great for backpacking. It does become second nature to walk carefully so you don't hurt your feet. I don't even think about it anymore. It's painful to walk behind people who wear shoes and watch them step so carelessly and nearly twist their ankles all the time. If Marrel Barefoot are so minimal you have to watch your footing, it'll be second nature in no time.
So, as far as Merrel Barefoot, I think the whole minimal aspect of them is a non-issue. What I would be concerned with is if they are worth the $100 and will they last and give you decent traction on the kinds of trails you like to hike.