I have gone direct onto T&G floor boards quite a few times.
1st was my own bathroom which is very small, The boards were checked and if any creaks or groans/loose boards found then they were secured. I laid the flor with 2 part Fast Flex, grouted with single part flexi grout....NO PROBLEMS after 4 years.
From then on i will lay using the same method ONLY if the client insists. WHY? because i believe in the overboarding method with Hardibacker or No More Ply. That method gives me more confidence.
I will never use Ply, the reason being is that Ply comes in different grades and can be liable to have bad batches. This info was told to be by various people in the woodworking trade including Wood supply shops.
Saying all that.....over the past years i have used all methods and never had a floor come up or even a grout joint crack....not that i know of anyway.