jam bo: I don’t think the limitations are any more than the completely obvious one – ie that if you crash you might squash the little one between you and the bars. That’s fine if you’re aware of it, but I seem to remember the bloke who made them denying there was any chance of that happening. I don’t think that helped his marketing very much.
But like I said, the people I know who tried them liked them. It’s a good solution to taking kids off road, you just got to be aware that the child is exposed to all the usual off road riding risks (including the one of having a big heavy thing (ie you) flying about in the even of a spill)