I'm a big fan of Gtd since 6 years. I used to use CulturedCode Things, Evernote, Dropbox, Minco(time tracking) and Mail.app.
After some years of use and multiple spikes into other setups, I migrated to using Emacs org-mode with git and mu4e for everything. It's cross platform, text only, does have a stable UX and API and the licenses don't expire all the time. Couldn't be happier!
I learned that my mobile is (to me) mostly a distraction and sometimes good to look up things. Actually moving the tooling to the desktop was therefore a good thing.
But: I definively can look things up from org through an app called MobileOrg. Also I can put things into my inbox. Mail still works the same with the built in mail app.
After some years of use and multiple spikes into other setups, I migrated to using Emacs org-mode with git and mu4e for everything. It's cross platform, text only, does have a stable UX and API and the licenses don't expire all the time. Couldn't be happier!