The ECM should flag errors on any attached microprocessors and they are pretty good at finding bugs in themselves. It is usually the bottom call in the MAP. The reality is they don't fail often enough to be a huge concern and when they do it is going to be a dead soldier, an input line receivers that failed or have a bad output driver. A meter or maybe a scope should show you that. Most shops don't use scopes, not that kind anyway. People, particularly shade tree mechanics and old timers just have brain freeze when they think about computers. The ECM makes it easier, not harder. The states trust them enough to have abandoned tail pipe inspections, trusting the ECM will tell them if a car is not running right and polluting. That usually also results in performance issues. I wouldn't open the hood for an engine problem that isn't leaking something until I scanned for codes. Same with my outboards.