The N900 was more or less a tiny computer running Debian. With 256MB RAM, and swapping on flash.
It was way too low spec to run reliably like that, you quickly ran into swap death. And it had none of the niceties of Android's memory management, having apps designed to be stopped as needed.
Security-wise it was also bad, it was just a normal Linux box, so banking apps would be a terrible idea.
If it didn't get killed, I wonder how would they have polished it up for public consumption.
And it had a Keyboard! With really real keys!
Yes, on Linux you can replace binaries and libraries in use, but then you're not actually running the new code until you restart the program and are still vulnerable to any security issue that it fixed.
And with things like runtime loading of plugins that now may be incompatible, and programs not expecting stuff changing underneath, online updates can be troublesome.
The online model works well enough for a command line usage where applications are transient, or a server usage where you remember to restart a service or two. But for a desktop user with long lived, huge apps like a browser it's not that good of an idea anymore.
I am aware of them - a friend worked on the N900 - but they were so far off 'phones' I didn't bother.