We’ve gone through SMS in New Zealand. I would say about 90% of the benefits come from 20% of the system, and the rest is all cost in time and money (and all cost is a harm to safety). We were promised from the start that it would be scaled to the size of our operations, but in the end we had to do all the requirements even if the CAA guys acknowledged it made little sense for us.
The hardest part of implementation was a lack of understanding from CAA about SMS since they were quite new to it as well, so we had little effective guidance (despite a lot of effort on their part) and a lot of inconsistent directives. Different CAA officials would have different ideas about how parts were supposed to work, and we would get whipped back and forth or have to push back against some of the zanier ideas.