For the normal roadie, used strictly to tarmac, this sort of mixture of surfaces can be definitely challenging. No doubt experienced mountain bikers will smile when I say that for a roadie any deviation of direction taken by the bike is alarming. If the front wheel moves from under you then a crash is likely to happen, if your back wheel loses traction you may well fall off.
Off-road, though, is different. It involves almost constant deviation. Your wheels are rarely going in the same direction, its best not to be too precious about where your front wheel is going - just so long as it generally in the direction you want. If your back wheel is sliding left and right that's not a problem - just keep it turning around and everything will be alright, probably.
Here's a couple of events I've reviewed for Cyclosport:
Wiggle Wildwood 80km CX sportive
Evans Ride It Hampshire Sportive Cross