Well, I don’t have a lot of details at hand, but I have heard arguments to the effect that:
- the arupas were taught by the Buddha’s former teachers, so they’re not really Buddhist.
- (Others argue that what they former teachers were teaching were not really arupas. Which is equally implausible.)
- the arupas are not in right samadhi (and the five faculties, etc.)
As to the first argument, the former teachers first taught faith, energy, mindfulness, samādhi, and wisdom. So are we to argue that none of these are Buddhist either?
Obviously the Buddha respected his former teachers, and was quite happy to draw upon their ideas and practices. He rejected them, not because he had a problem with the practice, but for the reason stated quite explicitly in the text: because that “dhamma” (i.e. the teaching or system as a whole) leads only to rebirth in the formless realms, not to the ending of rebirth.
‘This teaching doesn’t lead to disillusionment, dispassion, cessation, peace, insight, awakening, and extinguishment. It only leads as far as rebirth in the dimension of nothingness.’
‘nāyaṁ dhammo nibbidāya na virāgāya na nirodhāya na upasamāya na abhiññāya na sambodhāya na nibbānāya saṁvattati, yāvadeva ākiñcaññāyatanūpapattiyā’ti.
As to the second argument, this is the problem I identified in my “Early Buddhism and Theravada” article of thinking of the path as 2-do list that you have to check off, rather than a spiritual journey that one undertakes.
Any particular teaching is merely a summary, which identifies a few key points. But the path itself is rich and complex, and can’t be fully captured even in the canon as a whole, much less in any one formula.
There are countless things that are taught throughout the suttas, yet which are not found in the bare lists of the eightfold path and other key doctrines. The Buddha, like any teacher, had to find a balance between clarity and comprehensiveness.
Some things may be necessary for some people and not others. Some things may apply in certain life circumstances and not others. Some might not be really necessary, but still helpful. And so on.
Clearly, the arupas are not felt to be a central requirement of the path like jhāna. Yet they are still taught very often, and in many contexts. And where they are taught they are firmly placed within the context of progress on the path, not as something extraneous or dubious.
I’m looking for a little nuance here!
Anyway, there may be other arguments in this space, but this is what I’ve got right now.