Rick ... I used to work on a lot of hydraulic systems and the biggest disadvantage we saw with the water cooled heat exchangers was when the failed .... you got water in the hydraulic system , and that was not good . The advantage to a water to
oil exchanger was that it is more effective than an air to air unit . With modern materials and construction techniques , they are pretty reliable now ..... millions of cars have used that method to cool the transmission
oil with very few problems.