I did a CI fellowship and they are all very different. Some are more academic focusing on research and can help you advance an academic career, some focus on operational work and can fast track you to admin, and some get you integrated into the startup world. The reason they really do need to be 2 years is that the lifecycle of the projects are anywhere from months to years, so you would not learn the health system, nevermind get the exposure needed to lead a project in just one year. However, the clinical commitment and required work varies from program to program as they may be funded differently.
For me, I did the fellowship and the way it was structured I basically made the same as I would as an attending with all the benefits of being a W2 resident. My fellowship was funded by the IT department, so all my clinical work was paid like an attending. Each fellowship is funded differently and there may be some flexibility, so if you are pursuing it, if financials are a concern, it is something to consider when evaluating programs.
Depending on your goals it may or may not be worth it, but it is exceptionally rare to come out of fellowship as a CMIO these days. That takes years of building relationships to be able to affect the change needed to be trusted in that role. Most graduates look to get 10-30% informatics (+/- a title) or assistant medical director time as a buy down from their clinical work and if they succeed there, they move up into ACMIO roles, medical director roles, etc. Some do go off into the startup world, but not sure how much CI helps them there.