When I try using Hummingbird's minification functionality, it gives me the following error below each JS/CSS file (in the File Details column): "Error: Error saving compressed file"
Also, when I initially installed Hummingbird, it could not create the "wp-hummingbird-cache" directory in /uploads/, so I had to create it manually.
Finally, I could only install Hummingbird (and other WPMU plugins) via the native WP plugin installer, not via WPMU Dashboard (where it produced an error about correct permissions).
Two things that I guess could have caused all this:
1. Folder/file permissions (most likely): I have folders set to 755, files to 644. Which permissions would Hummingbird require - and for which folders?
2. FTP/file access: we have disabled the direct FS_METHOD in wp-config, plus FILE_EDIT is disabled as well. So for instance, we have to include the FTP password when installing plugins (and we do not have FTP_USER/PASS/HOST declared in wp-config)