Getting rid of long discourses from Tipitaka, what could be true intention? I have heard that people mention long discourses (Diga Nikaya) are not authentic Buddha's discourses (not Buddha's words). The fact that makes me surprised much more is that they say "in Diga Nikaya, Brahmajala Sutta was not discoursed by Buddha". 

After I have studied Brahmajala Sutta; such bold words and complete coverage on Indian Brahman and Brahmin religious concepts as well as all modern religious concepts, I thought those kind of very complete specifications of 62 concepts/beliefs could only be named and discoursed origins of them by only Sammasambuddha or someone like Sammasambuddha (who has very complete Panna without any rival can claim or find loophole/falsehood or incompleteness in his discourses). 


What could be the motive of such weak claims?   
Is it more appropriate to find any incorrectness or loophole of Brahmajala Sutta and other controversially claimable Suttas in Diga Nikaya first before saying authenticity of these Suttas?   
Could it be more easier to prove by saying specification and origins of 63 beliefs that was not covered by Brahmajala Sutta? 
Is it correct way to say that Suttas from Diga Nikaya (start from Brahmajala Sutta) are ehipassiko? 
At the time of Buddhahood of Gautama Buddha, all India regions are overwhelmed by Brahmanism and Brahmins. Buddha has only 60 monks which are from rich men of Yasa young guy and friends. It is not inappropriate to convert some Brahmins who had a sense that Atta of Brahmanism was in vain. It was obvious that Sāriputta and Moggallāna are disciples of Sañjaya Belaṭṭhaputta and they joined to Sanga society with 250 disciples of same teacher. DN 12 Lohicca Sutta might be counter question to those scholars who questioned authenticity of Diga Nikaya but actually it makes infinite loop about disagreement. 
Another possible reason about Bramhajala Sutta is Suppiya did blaming on Buddha, Dhamma and Sanga but the sutta did not mentioned what were details words of Suppiya and to what extent Suppiya did false-fully accusing we do not have chance to know. If a leader of such Sanga society did not properly repeal wrong accusation, what kind of leader Buddha would be. 
Sariputta claimed that Buddha's Sila was the best possible Sila a person can keep it clean, unbroken. In Brahmajala sutta, it is where we get a chance to understand/learn Buddha's Sila by grades and it is also role model to future Sanga nonetheless future Sanga has tendency not to keep as many as Buddha prefer to. 
Propagation of Anatta views on Brahmanism overwhelmed Indian region is bold move of Gautama Buddha and many adversary effects should had been expected (need citations here). Scholars can logically do reasoning about it. Immediate decision about not to do any discourses to anyone just after enlightenment was nullified by request of Sahampati Bramha, so it is inappropriate or weak claim that Buddha would not do driving less dusty eyes Brahmins to become monks. 
Gautama Buddha did offensive (if improper reasoning on his) remarks to Maghandi's daughter in Maghandi Sutta. But the explanation was Buddha had Karuna on Maghandi couples and only the daughter was completely out of picture. For this ages of valuation of women lives, this kind of remark will be viewed as very offensive to community and legal level but this was some kind of example that Gautama Buddha did speak if there was great benefit to save Brahmins from unfruitful believes of Atta view and/or Veda. 
It seems like scholars cannot appreciate Buddha converting people who believed Brahmanism but they failed to notice that such big Sanga society cannot be done by overcoming adverse Atta view, worshipping Brahma and caste system.
To make a blanket statement that the "DN was not spoken by the Buddha" would not be wise. Obviously, there are certain suttas in the DN that reasonably conform with what the Buddha taught in other suttas as verifiable truth. 

It must be noted the core principle of the Buddha-Dhamma is that is it 'verifiable', namely: sanditthiko akaliko ehipassiko opanayiko paccattam veditabbo vinnuhi. Any sutta that departs from this core principle is open to question. 

It must always be kept in mind that pre-Buddhist Brahmanism was very different to Buddhism however, over time, both Buddhism &amp; Hinduism changed so much that they became largely indistinguishable resulting in Buddhism becoming extinct in India. Many commentators have attributed this to Buddhist efforts to proselytize the religion &amp; the DN has been deemed (for example, by Bhikkhu Sujato; by Bhikkhu Bodhi, page 31 of his SN) to be the major vehicle of proselytization. In other words, it was corrupted Buddhism designed to seduce Brahmans. It is widely acknowledged that: 


  Recent
  scholarship suggests that a distinguishing trait of the Digha Nikaya
  may be that it was "intended for the purpose of propaganda, to attract
  converts to the new religion."




If the DN was composed for the purpose of propaganda, to attract
converts to the new religion, then it is unlikely the Buddha spoke for this purpose, since the Buddha was only interested in converting those with little dust in their eyes (MN 26) and keeping his doctrine pure (AN 2.23, AN 2.25, MN 22, MN 38, etc). 



As for the Brahmajala Sutta itself, it is not the specific sutta best chosen from the DN to make a claim the DN was not spoken by the Buddha because the principles in the Brahmajala Sutta appear sound. It is others suttas best chosen to make such claims, such as the Maha Nidana Sutta, which is obviously a corruption of the Buddha-Dhamma. 

This being said, the Brahmajala Sutta might possibly contain words or doctrines that are not found in any other sutta, such as saññīvādā, antānantikā, amarā­vikkhe­pikā, etc. I think only a competent scholar could make a reasonable hypothesis. If so, it is open to question.

Another peculiar feature of the Brahmajala Sutta is it is a response to a debate between the wanderer Suppiya together with his pupil, the youth Brahmadatta. It is possible but it also seems strange the Buddha would give such a lengthy, pedantic &amp; close to paranoid anal discourse due to the discussions between the wanderer Suppiya together with his pupil, the youth Brahmadatta. 

The paranoia or defensiveness gets even worse, with the morality sections referring to "Gotama" himself abstaining from attending unsuitable shows, games and recreation, etc, to the point of sounding very childish. At least I would expect the Buddha to refer to his Vinaya abstaining from these worldly behaviours rather than himself personally. 

To me, the Brahmajala Sutta sounds like a PhD dissertation rather than the words of a Buddha. That being said, as already posted, the principles in the Brahmajala Sutta are sound. Therefore, there is nothing explicit in the Brahmajala Sutta to deem it not Buddha-Dhamma. 

If one wishes to critique the authenticity of the DN, others suttas are best chosen. 



To conclude &amp; to answer the question, the reason for dismissing DN suttas such as the Maha Nidana Sutta is because they promote wrong understanding of Dependent origination, which is the core dhamma of Buddhism.