The stock iOS Mail app lacks the ability to add attachments other than pictures and video, or share/save as a PDF for example
http://forums.imore.com/ios-8/326147...-ios-apps.html
If adding attachments to new emails or in a reply to situation is not important, then the above is probably not a big deal.
I do like the native Mail app - and generally prefer native apps for core functionality like mail, messaging, browsers... and am trying out Outlook side by side for one of my IMAP accounts to see how I like it and how reliably it performs.
^^^ + what eb_therebel said.