This info is from pulling and running dozens of ECUs of each number discussed. for 240's and 7-series cars.
1) LH 2.0, 503 (0 280 000 503) and 510 used to be a constant demand item 5 to 10 years ago with a bad ECU resulting in "failure to adjust mixture" and horrible running. Those days might be behind us as fewer of these problems are diminishing.
2) Now, the bad ones are all LH 2.4 including the 561, with few pink label ECUs surviving. Typically, this is a no start failure. However, I have had two white label 561s that failed recently so they are not a sure thing. I have never had a bad 951. that I can recall. The 16 valve ECUs 928, 571 seem to work fine in these cars too.
3) The LH 2.4 California emission with EGR 556 Volvo original must be atrocious too. Every one I have seen lately is a Fuel injection or Programma rebuild with the old Volvo ECU long dead. The rebuilds seem to be just fine.
4) The only turbo ECU that causes problems is the LH 2.4 563, see Scott Cooks recent travails. That one has a steady demand so failure must be fairly common, though not like the non-turbo counterparts as far as I can tell. Possible replacement units are 937 and 967.
Other ECU failures may rarely occur but they are much more unusual than those listed above.