There's a pretty big difference in the flexibility you get when setting up synchronized shift on 6870/9070 vs newer versions of Di2.
I asked Shimano about this once, but they couldn't give a good reason for this.. other than an oversight .

Also... Synchronized Shift settings or profiles are tied to the cassette/crankset that were selected at the time the settings were created. Change either of those in the app, and you won't be able to use synchro profiles used with other settings