If anyone is in doubt about what a vendor or mfgr uses for materials, I'm sure a dealer of those machines can set you straight.
Getting back to the main thread topic, I can give my own experience after working extensively with 304 stainless steel. It certainly imparts no taste to water, and is extremely easy to clean coffee oils from. Brass, aluminum and copper can all give up some amount of ions into water, but this depends on the water going in as to how much. Can it be tasted? Again, highly dependent on water quality and sensitivity of individual palate.
As far as scale goes, I'm not sure how much less if any a stainless boiler would produce, but if you have hard water, it will scale up regardless. In a traditional espresso design, much more scale will be on the heating element than boiler walls. So maybe a stainless element is more critical.
To me, for espresso and on most machine designs, I would give 304 stainless the following pros:
- No change to water quality theoretically - for this to be true, all elements that touch hot water must be stainless (boiler, sealants, fittings, tubing, heating element, group head, shower screen, basket...etc). Also gaskets and rubber seals would ideally be silicone to match this ideal.
- Easy to clean oils - safer to descale - possibly builds up scale at a slower rate
- Insulates (not as conductive as brass, alum or copper) - why a pro? this is ideal if you are using immersion heating elements as opposed to conductive heating method like a Caravel. Less insulation around boiler would be needed for same efficiency.
- Doesn't corrode, rust or degrade over time as bad as other materials, but it's not impervious, it is called stainLESS after all
- Extremely hard to work with - this means..expensive - difficult to weld (or possibly can be silver solder brazed) - or joined with high quality fittings and hardware
- Stainless fittings are $$$$, at least 10x more expensive than brass, these add up quick on pump machines that have complex hydraulic systems.
- Every element stainless? Thermo-couples, elements, fill sensors, valve assemblies, solenoid valves...etc. There's a ton to think about on traditional machine design, and I'm not even sure if every part is available with a true 100% stainless material that touches water.
- Stainless tubing may not seem too expensive, until you realize it takes some serious labor to cut and bend properly, and aligning the tubing to fittings is critical to not put extra stress on them. Flaring can be done and flared fittings used, but add plenty more labor.
Even with expensive machines, I would doubt that your water path can realistically be 100% stainless. Design trade-offs must be made, and mostly that is just life and engineering. If you want to see what something close to 100% stainless as I've ever seen on a traditional machine, check out this guy's build:Custom espresso machine build
I can't imagine what it must have cost him, even if you discount his labor to nothing for the build.
You can certainly just use a stainless boiler, element and common brass fittings with copper tubing. Assuming also a brass group head. I would ask myself, how much less minerals are you really getting? And how much is that worth in terms of cost to you? Can you even taste those minerals? Can you alter your water quality to compensate most of the way?
Health concerns is something I didn't address above, some people like stainless for that reason. Brass has a very small amount of lead (called lead-free brass) in it to improve machine-ability (even 303 stainless does as well for the same reason, 304/316 do not). Copper is usually 99.9% pure, with phosphorus making up a small remainder percentage and other impurities. But in the end, how much does it matter? Many of the benefits of stainless, especially in traditional espresso machine design, are mostly theoretical, and because the complex system usually can never be pure stainless, I feel it's hard to make accurate comparisons.
Overall, I don't think one metal is clearly superior. They each have their trade-offs. And focusing just on boiler material I don't think is an exact answer either. To me, the whole system should be looked at, or at least the hot water system. Cold water feeds aren't as critical.