Most people who use the terms frequently seem to think extremism is a bad thing, but they rarely say what it is that they are denouncing as bad.
It is the custom of some Christian bishops to sign their official correspondence with a self-designation like "the humble mediocrity", or words to that effect, and this might give the impression that the Christian faith is anti-extremist, until one reads the words in the Revelation of St John the Divine:
And unto the angel of the church of the Laodiceans write; These things saith the Amen, the faithful and true witness, the beginning of the creation of God; I know thy works, that thou art neither cold nor hot: I would thou wert cold or hot. So then because thou art lukewarm, and neither cold nor hot, I will spue thee out of my mouth. Because thou sayest, I am rich, and increased with goods, and have need of nothing; and knowest not that thou art wretched, and miserable, and poor, and blind, and naked: I counsel thee to buy of me gold tried in the fire, that thou mayest be rich; and white raiment, that thou mayest be clothed, and that the shame of thy nakedness do not appear; and anoint thine eyes with eyesalve, that thou mayest see.
Whatever the Church of Laodicaea was back then, it certainly could not be accused of extremism, but its lack of extremism brought not praise, but denunciation. And people like Christian saints seem to have bee characterised by extremism. Most of us mediocrities admire them, but do not really seek to emulate them. That would be over-the-top. The safest position is a moderate disapproval of evil, and a moderate approval of good, but nothing extreme, you understand.
Being extremist is, in fact, neither intrinsically good not intrinsically bad. It is not being it extreme that is good or bad, but rather what you are being extreme about.
Extremism and extremist are weasel words. Avoid using them, and be wary of people who use them too much. Those who are extremely anti-extremist are themselves being extremist.