Sort of stating the obvious to someone who has already considered it but the biggest problem is seeing the various students and of course Ueshiba M. and Takeda themselves at comparable times. It is really hard to make comparisons especially when some are long dead and legend has taken over.
What were Takeda or Ueshiba really capable of doing - was Shioda in his prime that much less then Ueshiba? Or conversely were certain student's capable of doing things that Ueshiba could not?
I would like to think that if one produces students that gain a wide respect beyond their own limited area art than you have been a successful teacher.
Of course it's hard. But at some point you have to take a look around and, based on your experience, make an opinion. It's not a thing that's ever likely to be proven objectively, but I think that it's important for all of us to try and make the assessment.
There are a number of people who have wide respect - but not much in the way of skills that I'm talking about. So, you could say that they are successful, and that their teacher has been successful - but I don't.