windvanes "work" to some degree, and there is no scale or standard test to compare them. Hence anecdotal evidence may be misleading but is all you have.
If you want a vane that can handle steering boats with higher steering forces relative to wind speed then amplification of the wind force via a servo pendulum is desirable. Hydrovane drives its own auxiliary rudder direct from the wind power and does not use the force of passing water
to amplify it. While it works well, it just isn't as powerful as a servo-pendulum. It "works" but whether it works as well in as broad a range of conditions is doubtful. A 40ft steel boat should be near the extreme of performance for the Hydrovane, so I suspect it has a smaller range of performance.
I don't mean to sound negative about them. I bought, and still have and use, a Hydrovane from the original English
inventor. But it converts the steering in a reverse direction to the standard unit, and it has a tiller arm and cross-link to a trim tab on the trailing edge of the rudder. It is not a servo-pendulum but it does use the force of the passing water
to amplify the wind force. It has been very reliable and effective. Sadly the new Canadian owners did not decide to manufacture the reverse conversion, or the smaller model of Hydrovane for which there is now no source of parts
For my money
the Windpilot is hard to beat. I preferred the all-stainless (but very light) Fleming
made in Australia
but that seems to have ceased production, or at least is under a different name. Both had the very desirable feature that the pendulum could be quickly and easily swung out of the water for entering harbor. Most servo-pendulums, like the Monitor, have to be locked to straight ahead as the pendulum is not easy enough to remove/replace daily. The Hydrovane also has to lock its rudder. Leaving in the water may not be a big deal but all things being equal I would prefer it out of the water when not in use. Another company, Voyager, makes a vane in Canada
similar to the Windpilot which might be worth checking out (I can't tell if it swivels up as well). There are several others made to similar designs. For a heavy 40ft boat I would consider the servo-pendulum/auxiliary rudder combined systems, such as Windpilot offers.