I know nothing about it; but it seems to me the 'problem with ki' is not in ki itself; rather from the semi-mystical nonsense prevalent in the world today. I hope I make sense at this to make my point:
I'm a serious pragmatist when it comes to self defence; when I started Aikido I was extremely skeptical about ki and its application. Now I have no reservations; perhaps my association with the Ki Society causes me to think in ki terms more often than other styles; I don't know - I haven't taken any other styles. But in this post; I'm thinking about folks at the other end of the spectrum: those I call the Granola of society; the fruits, nuts and flakes. There's a lot of 'em; and they think some very weird things - one only has to look at everyone's favorite MA video - Yellow Bamboo - to see that. As the recruiter for our dojo; I've gotten some very weird E-mails from interested parties. Many seem to get their concept of ki from Dragon Ball Z and other realms; I remember one in particular my Sensei showed me in which the person stated "i beleeve i have the most powerful ki of anyone in the last hundrd yeers. i want to learn how to throw ki balls at peeple." Sheesh - but I'm getting off-topic. The problem with ki as I see it is that people are either too willing to believe in ki as Luke Skywalker's Force; or so determined not to believe same they reject it out of hand.
As a VERY junior instructor at the dojo; I try to show ki as a viewpoint; a way of moving and thinking about moving rather than a separate force in its own right. I could be wrong about this; but for the moment, it seems to work for me.