I like yacon. I like it a lot. So does Jeremy. He wonders in his comment below whether failing with yacon can be counted as a major achievement.
Time to fess up: I am an experienced murderer of scarce germplasm. I'm not ashamed to list yaconicide as one of my achievements. It comes with the territory, like the old adage of keeping livestock and ending with deadstock. Or all political careers ending in failure. Persevere long enough, with any crop and you will fail. The hidden subtext is, therefore: grow a diverse range of stuff - you might end up actually eating something.
Yacon likes a nice warm, sunny climate. It didn't get that here in our last, so-called summer in 2008. That said, the yacon grew and produced a reasonable crop whereas other, supposedly hardier crops did much worse. Perhaps it is merely more resistant to the kinds of horticultural abuse I hurl in its direction than are other plants.
Yacon failures I have experienced include: offsets for replanting rot, shoots get frosted off, slugs bore bloody great big holes in the stems which then keel over. I think some sheep ate it one year, but I forget.....
The offsets for replanting can be a tad temperamental. If they dry out fully, they die, pure and simple. Keep them too wet and they rot, which is merely a slow, lingering death involving slicing soggy flesh from the end furthest from the buds, checking daily and repeating the process until, hey presto, nothing is left.
I find the best way of storing the offsets is to dig a hole into some well-drained ground, deposit them below the level to which frost will penetrate and then exhume them in the spring. They seem to stay healthier that way. Don't tell the resident vole population and you might have nice plump shoots ready to be potted up in the spring. This stunning innovation which I developed over a couple of seasons of trial and error (I forgot to dig them up), was independently discovered a few thousand years ago by Andean farmers.
So to answer your question Jeremy, failing with yacon is neither easy, nor is it particularly difficult. If you haven't failed yet, keep practising and I guarantee you'll succeed.