I've been shipmates with 3 different gears on 4 boats.
The Albin Ballad and later my Contessa 33 (not 32!) had the same Monitor unit fitted. Once set up and tuned it worked remarkably well. With the Ballad, whilst sailing double handed, we missed winning a RORC race, our class and overall by 40 seconds. The Monitor steered, and we trimmed the kite like demons.
The unit was then fitted to the Contessa for more double handed racing and a two year Atlantic Circuit, on which it never missed a beat and held the speed record..
I've sailed a season on a S&S Swan 40, which had a Pacific Plus. Reasonable gear, but poor in light conditions; it needs 4 kts of boat speed and 10kts of wind to perform. The basic construction using bushes rather than bearings doesn't help. It acquired the nickname, The Sulky Hun.
My current boat, an Oyster Heritage, long fin and skeg 37' designed by Holman & Pye came fitted with a Hydrovane. After 20,000 miles my views are: The basic problem is that getting the wind alone to turn an auxiliary rudder means it isn't very powerful. It's especially poor in a quartering sea, as the rudder doesn't have the strength to keep her running straight. Forget setting a spinnaker.
The Mickey Mouse way the rudder is attached means manoeuvring in a tight marina is fraught as it can't be lifted out of the water, unlike a pendulum servo gear.
Also, the people who sell it aren't very nice to deal with. I had extensive correspondence with them about the Hydrovane's poor performance. They recommended that a new larger rudder and a suitably strengthened shaft was the answer. Many photographs and accurate measurements were taken.
When the new parts arrived, it was obvious that they were never going to fit.When I queried this, they said "Oh you must have the 1.25" shaft, it won't fit"
Also, the new rudder was badly warped.
At first they refused to take the stuff back, "you ordered it, we supplied" they said. They changed their tune when I pointed it out it was their boss that specified the alterations in the first place. Nevertheless, they refused to refund my not inconsiderable shipping costs to and from Florida.
All in all, if I wasn't selling my boat, I'd remove the Hydrovane and flog it to some unsuspecting mug and fit a Monitor.
Sorry for waffling on.