Not all hardware. I couldn't even get the install disc for 10.04 to boot on my 2010 13" MacBook Pro. I'll give it another try when 10.10 comes out, but Ubintu definitely does not "just work" on all hardware.
In my interesting computer collection, I have a couple IBM PPC/POWER machines. When I acquired the first one, it took me some work to procure a set of AIX disks (DDS tapes would do) that would install on the machine. Several friends of mine suggested I should install Debian and be done with it, but I felt that would make them less interesting and opposed it. When I finally installed AIX, I felt they were complete, with the system they were designed to run, it all its CDE glory.
Last time I plugged a Mac keyboard on my Linux netbook, not even the function keys worked. It muted the speaker when I tried a search...