In the Revenge of the Sith novelization, after Order 66, Yoda and Obi-Wan Kenobi are discussing who will confront which Sith Lord. Obi-Wan wants to go after Palpatine, since Anakin/Vader is his former Padawan and friend. And then, Yoda says :
Strong enough to face this Lord Sidious you will never be. You will die, horribly and painfully.
Note that Yoda doesn't say that Kenobi is simply not powerful enough now. He definitely and irrevocably states that Obi-Wan would never ever have a chance against Palpatine. Kenobi is younger than Palpatine and they are both humans, he became a Master relatively recently, he is capable of learning advanced techniques like Force Ghost, he displays great wisdom and calmness ... yet according to Yoda he will never be good enough .
This raises the question, what is Yoda's view on Force sensitivity and Force abilities? Does Yoda believe in midi-chlorian count? According to Yoda, is Force sensitivity simply innate, and you cannot go above your level no matter how much do you try or train?
Is that the reason that he does not train Force sensitive individuals after the Jedi Purge, but simply stays in exile and waits for a candidate with a high enough midi-chlorian count (i.e. Luke)?