You can free motion on a horizontal or Drop-In bobbin machine but the thread path isn't as forgiving as it is on a vertical machine. The thread has to turn a corner - so to speak - before it is picked up by the hook; this can cause uneven stitches, dropped stitches or looping on the back of your project.
I have 3 Drop-In bobbin machines- a Singer 404 straight stitch with the card table, a Singer Spartan that I want to turn into a handcrank, and my Brother PC6500 which was never intended to FMQ and you can't make it do it either! The other 7 or 8 are all vertical machines. I have a Singer 31-15 industrial with the feed dogs removed and a straight stitch plate on it, if you want to FMQ in a hurry - hoo boy!