I had a Hurth ZF 50 on our Ericson 30+, about 800 hours on the 5416 16 hp universal diesel. The transmission began to slip at unpredictable times. I changed the fluid several times, occasionally it would turn darker than usual after even a few engine hours. Wanting reliability I decided to replace it--I had read that they aren't reliable even after repairing them. The mechanic agreed and recommended the Twin Disc 340 transmission.
Installation was fairly straight forward, though I had to reverse where the cable attached. Since then (four years ago, about 900 engine hours), the Twin Disc has also slipped a few times, but much less often than the Hurth. I have switched to Castrol Transmission fluid and it hasn't slipped since--I don't know if that is coincidence or if it will slip again.
I spoke with the Twin Disc company when it did slip and they had no answers as to why--suggested I was using the wrong fluid, but I wasn't, etc.
So I'm not a huge fan of either the Hurth, which has a terrible reputation, or the Twin Disc which also doesn't seem to be as reliable as I would like.
I baby the boat, so neither is due to rough use, and I do read the manuals to ensure I'm doing what's recommended.
Hope that helps!
Frank