I don’t use a lot of nickel, so I’m not really on top of what it produces when washed with mercury. I do have nickel deposits, but I haven’t really mined them beyond getting some cobalt early on.
I do, of course, have a mercury bath setup. In fact, just recently I set up a separate centrifuge for bulk-centrifuging redstone, and a big tank to hold the mecury. Prior to this I only had a very small (48 bucket) tank for mercury, but I was very short of chrome, and I had large reserves (2000+) of redstone, so I wanted the redstone -> ruby dust -> chrome process, but I didn’t want to just throw away the extra mercury.
Near as I can tell, mercury has little value beyond ore washing. You can “burn” it for power in one-shot batteries, but that seems like a waste. It’s also a fuel for Magic power converters, again a bit of a waste. The extra copper, gold, and silver you get from mercury washing seems a lot more valuable.
Up until recently, the only use I had for platinum was carbon fiber. Which it produces at very low cost (1 platinum = about 70 carbon fiber). When I looked into wireless redstone transmission to control the turbines, I discovered that those covers require EV emitters / receivers, which in turn require platinum rods. You can run through a lot of platinum in a hurry in you go into wireless redstone logic in a big way.
I ended up going wired for the turbines, which you can do now that multi-block casings transmit redstone signals. Still, I’ve got the wireless stuff in mind. Wireless redstone is potentially very convenient, and I tested that it works in / out of Compact Machines. I could, for example, broadcast the state of the steam buffer wirelessly, and every control circuit that cares about that could listen to that signal.
I also see that platinum is the only -1 EU/meter cable for 8kV. Clearly it’s way too rare and expensive for that. Unfortunately there’s no cheap alternative like aluminum. I guess that most people use tungsten steel cable, which is -2 EU/meter, but much easier to obtain once you’ve got a tungsten deposit.