There are some things that are frustratingly incomplete in iOS:
Deleting Completed Reminders: After a while, completed reminders start to number in the tens of thousands, and there’s no easy way to delete them. I finally had to make a shortcut to do this, but it should be part of the application. Other Reminder apps have this ability, but not Reminders.
To be fair, the Mac version doesn’t either. However, I can select completed reminders as a group and delete them. Something you can’t do in iOS.
Contacts by Company: On the Mac, you can mark a contact as a company contact. It will list the company’s name when you see the contact, and it will alphabetize by the name of the company. iOS follows this, but doesn’t allow you to modify or create a company contact card without reverting it to the name.
Contact Groups: You can create a Contact Group on the Mac, and in iOS, you can use the group contacts, but you can’t create or handle contact groups.
These seem to be minor issues which could be easily fixed in iOS, but aren’t.