"arguably" is the word here. There are those who ARGUE that tohei sensei was his best student (being the only one to receive a 10th dan even though the tenth dan was given to him after o sensei's passing) Others ARGUE that Shioda sensei was the best student out there. Still more ARGUE that Kenji tomiki was the best. And the "amount of time under the founder" has nothing to do with "how good a student" someone is. Just because someone spends more time training doesn't mean they're going to be better than the one who spends less time. To many variables factor in. This is precisely the point i'm trying to make.
I'm using tohei sensei as an example of an exceptionally gifted aikidoka who DIDN'T take the uchi deshi route and was still a phenomenal martial artist that SOME consider to be the best student o sensei had.
You learn a lot lot more and hell of a lot faster than normal training
While i would tend to agree with this statement I can't help but think of the people who have to come to my dojo who are ridiculously un coordinated. Uchi deshi type training isn't the best way to go about things for everyone. I can picture many un talented people struggling day after day and only getting frustrated because they make no progress even though they train 6 hours a day. Everyone learns differently.