i haven't myself really felted anything but i believe that it's easier to check to see where the felting stage is in a top loader
depending the yarn and the strength of a washer, detergent, water temp.. an item can go from nice and perfectly felted to a small shrunken mess quite quickly
so opening a front loader often to check on the progress of an item may prove to be really messy
i've heard it could be done in a front loader..
i guess most wouldn't reccommend it for a first try though
i found this article about hand felting in a bucket