Why do you not recommend front hub motors except on trikes?
They may have safety issues: front hub motors are not recommended except for use with solid, steel front forks. If your bike has alloy, aluminum, or suspension forks, you must use a rear hub motor instead. Installing a front hub motor on one of the above styles of forks can result in fork failure, meaning the wheel could come loose and you will take an unpleasant trip over the handlebars! Please note this picture showing a fork dropout beginning to fail and spread! If you see this fork failure – stop biking!