Aluminum frames themselves are not bad for an ebike unless they are ultralight race frames. Aluminum is mostly bad for the dropouts (where the axle goes through the frame and fork) and the fork.
Hub motor axles have two flat surfaces you must lineup correctly in the dropouts. Because of the flat surfaces, the effective diameter of the axle is bigger than the diameter of the dropout, but the axle fits in because of the two flat surfaces. All of the torque of the motor it transmitted through those flat surfaces to the frame to propel the bike forward. If you have a strong enough motor, eventually the motor will round out aluminum dropouts to the point where there is no more flat surface to flat surface contact, the axle will spin freely and usually rip a lot of wire out of the inside of the motor, which is pretty catastrophic. Tightening the axle bolts tighter does not solve the problem in the long run, believe it or not. Steel's properties are such that this does not happen with steel. This problem may never manifest itself on a low power bike.
The solution is to use a "torque arm": a piece of steel with a cutout that fits snugly around the hub motor axle and its flat surfaces, and has an arm extending from it that is secured around the frame. These are hard to find and are almost always custom parts. I have seen anything from carved out blocks of steel to a chopped up pair of box wrenches and hose clamps used to make a torque arm.
Also, aluminum fork legs cannot handle the weight of a front hub motor in the long run. Please note that most high end forks actually have Magnesium lower legs (lowers), but they get referred to as aluminum and Magnesium is no better. There are many pictures of aluminum fork legs torn apart by the torque of a front hub motor. Solutions? Use a rear hub motor. Or only use front hub motors with a steel rigid fork. Or add reinforcement to your aluminum fork legs. I am not sure if using a double crown downhill style fork gives you enough strength. But they will throw the geometry of most ebikes way off, and you have to buy a pretty old one: new ones all take 20 mm thru axles.