There are a few potential issues with these units, though they may be effective.
As a review of data, published in a March 2013 issue of Current Osteoporosis Reports, points out, at high intensities vibration can actually cause bone fractures. That's a particular problem because osteoporosis is one of the conditions for which vibration plates are being considered as a possible treatment.
The authors go on to note that continuing exposure to vibration has also been shown to cause low back pain in truck drivers; circulatory disorders in construction-site workers operating machinery; and other conditions such as blurred vision, tinnitus, headaches and joint pain.
They also identify a number of potential contraindications for using whole-body vibration. These include pregnancy, retinal detachment, fresh surgical wounds, joint implants, cochlear implants or having a pacemaker. If you have these conditions, the authors advise, you should avoid vibration therapy entirely.