I am no expert on wind vanes, so take my comments on this topic with a splash of salt water
, but I do have some experience that influences my thinking on the topic.
I have used a Monitor to steer a sloop
for thousands of continuous miles in the Pacific with no problems or faults with the Monitor itself. Many hours of constant use, zero problems!
It was dependable and served the boat well and allowed me to enjoy the scenery (dolphins playing with the boat, albatross following, etc.) or to read a good book or just relax in the cockpit
with other crew or alone. This was using a jib
and main, no use of spinnaker.
The Monitor wind vane
is robustly built and highly regarded by all of the sailors I know that have used them. I would not hesitate to add one to my boat if the boat did not yet have a wind vane. It may not be the solution to everyone's problems or boat, but it does work well for many and there are many adaptions to many different yacht hulls. There is also an optional "Emergency Rudder" kit that can be added to use the Monitor rig or mount as a "jury rig rudder" in case of loss of the boat's rudder
If I saw a boat (for sale) that had a Monitor on it, I would consider it one of the more valuable "features" of the boat.