We have to differentiate between valid differences of scholarly opinion between the recognised Madhahib which come up due to multiple possibilities in interpretations of texts & multiplicity of hadiths on the one hand and invalid misinterpretations and distortions of the Deen by either denying the clear ayat and ahadith or interpreting them in such a way that is either not possible linguistically or that goes against all principles of interpretation.
If an individual scholar clearly denies agreed upon matters of belief or rejects Sahih Hadiths or distorts the traditional fiqh, then other scholars have a duty to warn about it since thats a serious matter and people may get misguided due to such a scholar.
This is not sectarianism. This is preservation of the Deen and a duty of the Scholars which is crucial. Allāh’s Messenger (ﷺ) stated: “This knowledge will be carried by the trustworthy of every generation. They will expel from it the distortions of those who go to extremes, the lies of the falsifiers, and the interpretations of the ignorant.” (Authentically Reported by Ibn Qutaybah in ‘Uyūn Al-Akhbār 2/119, Al-Bayhaqī in As-Sunan Al-Kubrā 10/209, Al-Ājurrī in Ash-Sharī’ah no. 1-2, At-Tabrīzī in Mishkāt Al-Masābīh no. 248, Ibn Abdul-Barr in At-Tamhīd 1/59)
The responsibility of the common people is to stick to the general body of Ulama of their school of thought whether Shafi, Hanafi, Maliki or Hanbali and not follow those who go against the general scholarly consensus. The problem is people get impressed by good oratory skills and then defend their favourite speaker / Daiee even when they have problematic positions.
That's because, many people cannot identify problematic opinions of some famous writers or speakers. That's why such individuals have a large following of awaam. Informing about their pitfalls is a duty of those who know about the issue. It's an amanah.
Just like when an individual doctor has a problematic treatment protocol which goes against the mainstream medical knowledge, then it's the duty of other doctors to warn the people against him. So, just because there's a controversy involving one doctor, patients cannot stop going to all doctors! Those who warn against him are not creating fitnah, rather he was the one who is creating fitnah and the others who pointed it out are only doing their duty and it points to their sincerity and well-wishing for the society. It also indicates that the system of medical practice is having effective and robust mechanism for preventing it's misuse and abuse by any erring individual. This mechanism should actually increase the credibility of the system and strengthen the trust of the people in it.