I feel sort of "lucky" and puzzled at the same time, but I totally don't understand why. Let me explain.
The requirements for FCP X, Motion 5 etc are: OpenCL compatible card.
I have an iMac from 2008 with an ATI Radeon HD2600, so not compatible. Furthermore, I read everywhere that MacPro users with the same card as I have could not download the app, the AppStore doesn't let them do it.
In my case, not only I could download the app (Motion 5), but it seems to run perfectly! I haven't had time to try all the features yet (I'm like many people here I can't do much if I can't import previous projects), but what I have tried does work.