I downloaded and have been using Oak for a few weeks now and so far, I think I like it. I haven't seen ads and the app is pretty basic. They have guided and unguided sessions and I find myself returning to the guided version even though I've pretty much memorized the instruction. The voices are nice and I actually find myself doing a better job of staying focused on my breath.
I don't know that it's been beneficial yet, but it is becoming a habit.
It also introduced me to Alan Watts and that can't be a bad thing.
For me it hits the sweet spot for simple, attractive, and useful.
While it generally costs more to live in the west than the east, it seems to upset a lot of new yogis that Yoga is not traditionally paid for transactionally in India, in the westernized model. Nor is India a country of 1 billion Yogi's.
I'm very happy for people who find any kind of practice, exercise, etc that improves their life. Excluding people from that on the basis of money alone is something that I have not been able to agree to agree with.
The difference is similar to parents who raise adults to remain children and subservient vs. parents who want to raise children to be adults.
The same can be said for "gurus". True gurus want to raise equals so the student surpasses the teacher.