We have 10 iMacs in the studio..... They are completely reliable workhorses and none of us would rather sit at a laptop with an even smaller screen and weaker config.
I don't doubt their reliability. My MBPs have been mainly reliable. And no, I wouldn't rather sit at a laptop either; that isn't what I'm saying. What I am saying is this: you give up expandability with all in ones just like you do with laptops; you give up portability with all in ones and traditional desktops. Each of the three systems gives up something. All in ones give up more. If I'm tied to a desk, I want a tower with all it brings (easy swapping or addition of components, multiple video card options, multiple hard drive options, optical drive options, etc). If not, I want a laptop. However, this obviously works for some people and that's great.
The upside is upgrading of storage without any disassembly, the downside is more clutter if you have several drives.
The upside is pretty minor given modern chassis design. It's really easy to replace drives. I run 4 internal drives, and keep two open SATA ports for when I swap out the main data drives (currently they're 4TB). Pulling the drives and putting in new ones is easy. On the other hand, I hate clutter. That's a big downside for me. I have an external drive tucked away out of sight which I use for temporary external backup, but my main backup solution is cloud based, so I don't need nearly as much external storage as internal storage.
The speed of Thunderbolt allows an external drive to perform to the full potential of its internals, whether spinning disk or solid state. Given a fast enough connection, there is no performance downside to the drive being external.
Out of curiosity, can you build RAID arrays over thunderbolt (heh, I think these days we can all ignore the "I" in that particular initialism).