The main problem with Beast Machines was that the Maximals' beast modes were at times just too ambiguous. With Beast Wars, even Transmetals and Transmetal 2s, you can still look at their beast modes and know what they're meant to be. The BM Maximals' beast modes were so abstract that Hasbro felt compelled to label them on their packaging!

^it's a bit hard to read here but right underneath "Optimus Primal" is the word "Gorilla." When you need to
tell people what the hell the alt mode's meant to be, then you know it's not a good looking mode!