That white paper doesnt say "dont buy aluminum". It provides quite in depth view on each metal and how it will affect boilers life.
Plenty of people are saying corrosion, grey goo, blocked lines.
https://www.google.co.uk/#q=stainless+steel+aluminum+heat+exchanger
Yes, aluminum does have 4.5x better thermal conductivity, but tube walls are much thicker due to fact that is not as strong as stainless steel - will crack and leak over time. On that fact, thermal cond. is equal.
Plenty of people are saying corrosion, grey goo, blocked lines.
https://www.google.co.uk/#q=stainless+steel+aluminum+heat+exchanger
Yes, aluminum does have 4.5x better thermal conductivity, but tube walls are much thicker due to fact that is not as strong as stainless steel - will crack and leak over time. On that fact, thermal cond. is equal.