Outside of its lack of graphics, exactly how is it awful?
Apple Mail is very reliable for the most part. It's some servers that aren't.
Mail.app should be a mature product by now. There were a lot of issues with Gmail with Mavericks until they ironed out the bugs.
Good luck configuring a Hotmail/Outlook.com account to work. One of the major e-mail providers on the planet and it still doesn't work properly with Mail.
The client itself is embarrassingly basic and lacks features. I don't expect it to be as feature rich as Outlook 2013 but I expect more than the minimalist approach taken by Apple with the app.
I don't understand why my e-mail arrives instantly on my iPhone but can take upto 5+ mins to arrive on my Mac. I also have to manually refresh to see some new emails in my inbox.
Unread/read e-mails do not always sync properly with other devices. It's only ever an issue with the Mac. I have a few Macs and they all have this issue.
My Gmail account is my primary mail account but I also have a Outlook account that I have to use a third party app to view my Outlook.com e-mail (Airmail) due to it not working with Mail.app.
It's a damn mess.