The early demo that some of the press got said the FMV is really…clunky. Felt unnatural, like you were watching a music video while playing your game rather than being 'engaging' like Activision insists it is. Also samey, since most songs won't have more than one live video.
On top of all that, it won't have the pre-existing DLC base Rock Band 4 will–RB4 will have something like 1400 DLC songs and will import your purchases from 1/2/3 (if the licensing is still valid), while GH Live will apparently have…none.
The only thing so far negative I can find about Rock Band 4 is the Keyboard and Pro Guitar got dropped