I've been able to capture the walls of an enemy with siege towers ( I do not use ladders ) and also the towers on the walls too.
No problem for me.

By fail do you mean don't make it to the wall, as in burn down before they reach the walls ?

That cna happen , if it does then your just unlucky,especially if a siege tower burns down to the ground.
Rams tend to burn down ( 95% in my games ) when assaulting stone walls or up.