Re: The Terrible...
"It's astoundingly slow at searching "folders" - far slower than grepping through a bunch of mbox-format files would be "
I suspect you haven't used Outlook in a long while. Searching my local Outlook 2010 mailbox on Windows 7 with well over 100,000 messages and many folders gives an instant response to any keyword - including in messages themselves.
"renders some non-text media by default in the preview and reading views. That's been a whopping security hole since the late '90s."
Outlook stopped rendering active content in preview about 15 years ago. I am not aware of any publically attacked exploits since then that automatically exploit Outlook with no user interaction.
"Splitting large message stores apart, for example to improve backup time, is a pain in the ass."
You don't normally backup Outlook locally - the data is on the server. But VSS can take a near instant snapshot if you want to backup locally regardless of the store size.
"Outlook sucks"
I have yet to use a mail client that even comes close, so would be interested to know what you recommend instead that could practically be used in SMEs?