Well, it's never really "OK," since galvanic corrosion starts the instant you attach two dissimilar metals. But, in refrigeration we do it all the time, driers, accumulators, compressors, etc..
The deal is, if ions are present (salt in solution for example) or any electric current (like water piping used for ground) the galvanic effects are accelerated drastically.
So, for best practice, keep unions between dissimilar metals to a minimum, use dilectric unions when possible, use galvanizing compounds (cold galvanizing spray or zinc rich tape) when dilectric is not possible, and be aware that steel to copper connectinos are a designed in failure. . . .eventually.
It's the chemistry that's key. I'm finding that the metals worry the heck out of people, but the liquid is the key ingredient. Most of the time, the fluid is benign, and no electrolysis even occurs to any degree noticeable.
If you master what the fluid is doing, the metals will last a long time.
I do a bunch of steam work, but I have to eat, too. Hot and cold water systems make up the difference in my time.
I agree, dielectric unions leak more than they protect, UNLESS the water or other fliud is a genuine problem.
Let me add that a system with NO dissimilar metals would last longer......
Longer than what?...That cast iron snowman boiler with brass valves and control wells and copper piping in YOUR basement, maybe? That's what, perhaps 90 years old? As old as the house? How old is your car? Your computer (I bet you have one)? your TV? Your microwave? Your nintendo?
How long will those other things last?
Sometimes, we worry about metal when we should focus on water chemistry.
Well, nothing lasts for a life time. Mr. Galvan of Spain came up with the idea of using dissimilar metals. Thus having an effect on metals. You can speed up or slow down the effects of corrision. Look at your boat tralier. Type K thermocouple. Two differant wires. Listen to Bama and Noel.