We were down to a choice of Hydrovane and Monitor and started doing the really detailed research. Because the Hydrovane steers the boat rather than steering the wheel like a Monitor it must have a much stronger mount and often transoms have to be reinforced. Our transom is very strongly built so that was not a problem. What was the problem that our boat was built with a variety of hoses (exhaust, bilge, fuel vent) looping up and then down inside the transom. Having to remove and reroute all of these would have been a nightmare. With the Monitor you only have to drill four holes and mount the unit with fender washers - whole thing took about a day. Monitor make an emergency steering option but I no nothing more about it than it exists. We have been very pleased with the Monitor and can recommend it.
Finished the circumnavigation in early February in Grenada. Have to work on a book project for the next several months so the boat will be waiting for next year.