Yoda didn't believe turning away from Dark Side is possible, and kept this belief right until his death. He famously said to Luke Skywalker:
Once you start down the dark path, forever will it dominate your destiny, consume you it will.
He may have changed this opinion moments before he died ("There is another Skywalker", i.e. Anakin Skywalker is still there somewhere in Vader). But this is beside the point. Assuming he didn't deliberately lie to Luke, at the time of Luke's training he didn't believe redemption is possible.
Yet he had example of abandoning Dark Side right before his eyes in the canon novel Dark Disciple:
Asajj Ventress (Yoda did meet her as Sith assassin in Star Wars: The Clone Wars TV series, S01E01). She came to Jedi Council while Yoda was present and formally declared that she abandoned Dark Side. She also asked for help to save her lover Jedi Quinlan Vos who has also fallen to Dark Side. To cut long story short, in the end Ventress sacrificed her life to save Vos and turn him away from darkness. Both of them were redeemed.
So, my question is why did Yoda believe that redemption is not possible when he had witnessed these examples of Force users abandoning Dark Side ?