Disadvantages of inserting iframe as html directly into text editor:
User cannot insert html into the editor in “Visual” tab. User should insert html only in the “Text” tab
Then after switching from “Text” to “Visual” tab in TineMCE the iframe html replaced with yellow square and user can not edit iframe params. User could edit iframe params only in the “Text” tab
Older WordPress versions removed (stripped) iframe because of the security reasons; New WordPress versions does not remove iframes; If you does not know this than I suppose you are pretty new in WordPress development; And if in future WordPress versions will remove (strip) iframes html again than the data will be lost;
iframe shortcode does not have these disadvantages and WordPress will never remove shortcodes.