My personal biggest gripe is the Gmail app on iPhone. There is no possible way to stop it from trying to force you to change your default browser to chrome every 2 weeks when you open a link.
the Mail app has its own massive issues. for whatever reason it is completely unable to just background sync and notify you immediately when you get an email. why is every messaging app able to do this instantaneously but email is not?
also their new default-on email classification system is a fucking nightmare and I was so glad when I figured out you can turn it off