[*vEdic*] karmAs advocated by pUrva mimAmsa, the principle of Ishvara established by nyAyA and dhyAna-vichAra practised by Bhuddhism which negated both karma and Ishvara, exist as different phases/parts of AchAryAl's advaita. However, when these schools of thought focussed only on a particular phase/level of [*spiritual evolution*] and got stuck at that phase itself, AchAryAl gave His school of thought which accomodated each one of them at its suitable level.
As other siddhAnthAs stopped at a particular level, He argued withthem too.
VaidhIka karmAs, no doubt, are acceptable to AchAryAl. The stand of pUrva mimAmsakAs that neither bhAkthi nor gyAnA is required, however, was not acceptable to Him. He advised, "The karmAs you do can not yield fruits by themselves. Karma is a jada vasthu. Therefore, the person who bestows the 'phala' for the karmas is only IshvarA. It is He who has ordained so many karmas through vEdAs, inorder to carry out the activities of this lOkA. The fruit of any karma done, therefore, should be offered to Him only. We have to engage in karmas, which are given to us, to be carried out, for the welfare of the whole world and offer the fruits thereof back to Him. Only we when do that, more than the direct fruit of the karma, due to the renouncing of the karma-phala, we will get the greatest fruit of all - chitha shuddhi [*purity of mind*].
Similarly, He argued with NaiyAyikAs, "It is not sufficient to establish the existence of Ishvara by logic alone. That logic has to result in direct experience. The direct experience of Ishvara has to lead to the realisation, in the end, that there exists nothing that He is not. The culmination of bhakthi, at its zenith, is the advaitic realisation that even we don't exist as a different one from Him" and established advaita.
If we engage always in the vichAra/dhyAna such as, "Who are we? What is the Truth ? What is dhukhA ? etc" overcoming the pulls and pressures of attachments, then we may also get the clarity/revelation such as the one BhuddhA had got. There is also a view point which says, "BhuddhA has not mentioned about Ishvara directly. Though he hasn't defined nirvAna exactly, he hasn't objected to Ishvara explicitly either. Therefore, it could be possible that he could've obtained the advaitic gyAna; only that he couldn't express it in words properly". Even then, is it possible for every one to engage in dhyAna and vichAra ? Does one not require to have pre-requisite [*spiritual*] maturity ? Yes, it is required. That is why vEdic dharmA has prescribed karma and bhakthi as a prelude (to gyAna). Only after obtaining purity (shuddhi) and one-pointedness (IkAkriyam) of the mind through karma and bhakthi respectively, one can engage in gyAna vichAra about the truth.
The vEdAs and smrithis have classified the karmas based on the maturity of varieties of jivAs and also for the welfare of the world and prescribed them as varna-dharma and Ashrama-dharma. Based on the adhikAri bhEdham - when such and such a section [*of the society*] engages only in such and such a vocation, not only they'll obtain srEyas but also contribute to the welfare of the world - various karmAs and upAsana mArgams have been classified and prescribed by them.
As Bhuddhism overlooked all these factors, rejected the vEdhOktha karma, upAsanA etc and prescribed dhyAna-vichAra for all without taking the 'adhikAri bhEdham' into consideration, the activities of the lOka would be disturbed; except for a rare one or two like BhuddhA, as others may not get enlightenment [*by such an approach*] AchAryAl condemned even Bhuddhism. As mentioned earlier, the condemnation of Bhuddhism, carried out in a massive scale earlier by both Kumarila Bhatta and UdayanAchArya was completed by AchAryAl!