I think per-activity configuration is a hard problem to solve, mainly from a user experience/workflow perspective. There are still certain parts of NVDA config profiles that constantly trip people up, for example, and many of them without obvious solutions.
Having said that, Apple's decision to explicitly define a subset of settings that can go in an activity seems like the wrong way to do it. It only leads to fragmentation, and the feature being far less amenable to creativity than it otherwise could be.