How coherent is Buddha-dharma if there is only one aggregate: form?
And no immaterial jhanas?
And no karma?
And how could one possibly define nibbana in materialist terms? The cessation of brain activity = nibbana?? That is a dire proposition.
So it's a pretty easy answer: it's not coherent at all.
There were materialists in the Buddha's time too - the Carvakas - and they were understood (by the Buddhists) to be about as heretical as one can get to Buddha-dharma.
In today's age, I would agree that there are some nuanced and sophisticated and well founded versions of materialism......but all of them are contrary to the buddha-dharma: the very logic of buddha-dharma is 'a mind that extracts itself from entanglements with form.'