Muhammad ibn Abd al-Wahhab takfired the entire Ottoman empire, called Ottoman Muslims "mushriks" worse than the Quraysh, and launched "jihads" against them, where he killed them in their mosques, homes, and marketplaces.
For this reason, Ibn Abd al-Wahhab was condemned as a Khariiji-like heretic by all the scholars of his era (e.g., Ibn Afaliq, Ibn Abidin, al-Sawi, the Ottoman Shaykh al-Islam) .
In fact, no one considered his followers mainstream Sunni Muslims until the early 20th century, when the Saudi state of Najd annexed the Hijaz and forced Wahhabi scholars to tone down their teachings on takfir.
It is this reason why Wahhabism has a bad name.
Admittedly the term "Wahhabism" does have a connection with Allah and the Quran. But so do the terms "Hezbollah", "al-Ikhwan al-Muslimin", "Ayatullah", and "Islamic State (ISIS)". Yet few Muslims, particularly the followers of Ibn Abd al-Wahhab, hold that such names cannot be used as insults. Indeed, Ibn Abd al-Wahhab's followers regularly insult groups associated with these names.
Admittedly, some Muslims use the term Wahhabism as an insult for those who defend proper tawhid. But most Muslims do not. Rather they insult Wahhabis for holding heretical and/or inconsistent views on a variety of issues.
Hence, Wahhabis are criticized for
- mass takfiring and killing other Muslims
-allying with non-Muslims to kill other Muslims after mass takfiring them (e.g., allying with the British against the Ottomans in WWI and allying with the US against Iraq).
-defending the stance of Ibn Taymiyya that it is acceptable to believe that hell is not eternal
-defending the view of Ibn Taymiyya that the world has existed alongside Allah for eternity
-defending the view of Ibn Taymiyya that it is permissible to call on jinns for protection and aid
-defending the view of Ibn Taymiyya that the Prophet ﷺ was temporarily tricked by Satan into preaching shirk (qissa al-gharaniq)
To be sure, Ibn Taymiyya is a great and respectable scholar. But the problem with Wahhabis is that they often assume his infallibility, and do not give sufficient respect to other great Sunni scholars like al-Tabari, al-Ghazali, and al-Nawawi. Indeed, Wahhabis demand tolerance or acceptance of Ibn Taymiyya's problematic views, while exhibiting zero tolerance for the views of other scholars, which reflects a type of chauvinism.
Contrary to what is claimed by Ibn Abd al-Wahhab's followers, many Muslims who never practice tawassul, istiighatha, or mawlid (like myself) oppose Wahhabis over their many other deviations, which sometimes rise to the level of kufr and merit strong criticism.
As for the term "Salafi", it was not used by the Salaf themselves or medieval Sunnis to denote the notion of following the Salaf. All Sunni groups have always believed in following the Salaf, so they did not need the label "Salafi" to mark themselves as holders of this position. This is why they all followed madhhabs established by members of the Salaf (Abu Hanifa, Malik, Shafii, Ibn Hanbal). At most, sometimes the term Salaf was used to denote the aqida of the Hanbalis/Ahl al-Hadith as opposed to that of other groups like Asharites and Maturidites.
The notion of "Salafi" as meaning following the Salaf was created in the early twentieth century by the modernist reformers Abduh and Rashid Rida, before being picked up by al-Albani and other scholars in Saudi Arabia in the middle of the twentieth century. They then started calling themselves "Salafi", as opposed to earlier terms like "followers of the Najdi/Wahhabi dawah".
Use of the term Salafi raises certain concerns. Those who use it today do not know its history. Moreover, the followers of Ibn Abd al-Wahhab use the term to falsely claim that, opposed to other Sunnis, only they follow the Salaf. This is a misrepresentation of Sunni views, and distorts Sunni history.
Beyond this, due to the influence of Abduh and Rida, contemporary "Salafis" regularly mock the beliefs and practices found among the actual Salaf, such as the notion that there continue to be a class of miracle working saints in all eras, that there exists forces of baraka, and that knowledge of the supernatural does not only come from scripture but also from direct interactions with supernatural forces (e.g., in dreams, waking visions, jinn possession incidents). And this is not even to mention the practice of tawassul which was endorsed by some major figures from the Salaf like Ibn Hanbal. "Salafis" make the bizarre claim that the actual beliefs and practices of the Salaf are later "Sufi" accretions which have nothing to do with Islam. In reality, Abduh and Rida rejected these beliefs and practices as non-scientific superstitions (khurafat), because they were modernizing reformists, and then their ideas were picked up by later "Salafis".
This is not to say that all Salafis are bad. In fact on the whole, many Salafis are quite good. These are the Salafis who are heavily influenced by Ibn Hanbal and Ibn Taymiyya (who never claimed to be infallible). But as for the "Salafis' who follow Ibn Abd al-Wahhab and Rida and Fawzan, then their views merit criticism.