I had a Hurth 2:1 box fail after seven years, the same symptoms, slipping in forward gear but OK in reverse. I'm not on the boat at the moment so I can't check, but I think it was branded ZF10M. Kingsbridge is 35ft, the engine a Beta Marine BD722, so only about 19hp maximum, and I guess about 8-10hp at cruising revs. I changed the oil (definitely ATF) at the specified intervals. It took a long time to fail completely, a year or two I recall, and in the beginning it would slip when cold, but behave when warmed up. I eventually got fed up with nursing it after an interesting trip back up the Weaver.
The fitter who changed it used the expression " Ten months or ten years, who knows?". My current ZF box is three years old now, and I'm not going to chance my luck by commenting further. So far so good.
Mike