Aluminium generally is stiff, quite uncomfortable for longer rides (100km). I own one, had a lot of fun with it.

Now I have a carbon road bike, don’t care about the weight, did care about comfort. Comfortable, flexible, but still has stiffness in the right direction (torsion is way less). Longer rides are much more pleasant.

There are also nice titanium bikes and steel bikes. From what I heard titanium and steel frames tend to be a bit more flexible, no idea why.