One of the biggest issues with mobile hydraulics is that every part of things change. Coils are warm. Coils are cold. Oil is warm. Oil is cold. Oil is old. Oil is new. Battery voltage is high. Battery voltage is low. Temperature is cold. Temperature is freezing. Temperature is freezing with old nasty oil. Filter hasn't ever been changed.
Then you get into the dynamics of the machine you are trying to control. Temperature, lubrication, structural soundness, and precision all come into play. If you are just trying to bang a cylinder to a stop, that's easy. If you are trying to swing a 100' boom 80 feet from the centerline without tossing the guy out of the basket, that's something else. In those cases, the difference between hot and cold outside matter more than you might be aware.
Even if you get it dialed in on a warm summer morning, that setup might be wholly inappropriate on a cold winter night, or with a different operator, or with a different hydraulic oil.
This is simply my opinion, but if you wish to be successful, you need to consider the whole package.
Another thing, there is a valve from a company called Apitech that runs at a very low frequency (like 30 Hz), and their technique does have some merit. You can see the levers pulsing in sync with the control signal.
In all cases, you want to run any coil at the manufacturer-intended voltage and frequency. Too low and it comes out as heat. Too high and it's hard to control predictably.
Good luck!