I provided my feedback on this issue and several others issues over 12 months ago (maybe even 18 to 20 months ago) when Office 365 was in beta and again when it launched. If it takes Microsoft that long to fix issues, then I'm making the right decision.
No, it is not because it does not have a GUI. Although, a GUI matters. Why don't you make everthing in Office365 require PowerShell... heck, I'm sure we can send email via powershell. The point is that a GUI allows us to quickly do things. Bottom line is that SendAs does not work as I require (and all the time wasted trying to make it work).
Not to mention that it took me several days to even get your powershell to work! I had to go through several obscure steps and then find out it does not work on Windows 8... something is wrong.
And looking over the forum, it seems there are still issues with validating a domain for use with O365... how many hours have people wasted with that nonsensical process that Microsoft refuses to fix?
I'm sure there are other issues that I did find. Yes, I have to "give up". I have to go with a solution that works out of the box and works now. That would be gmail.