There is a distinction between warning against a person in general for their creed, methodology or character, and not recommending them in a particular context. A person may be a Salafi, or even righteous and yet not be fit for a particular role at a particular time.  Not recommending such a person or advising against using them isn’t “warning against them”, it’s wanting good for the person advised or even both the person advised and the person he’s being advised against. In other words: naseehah.

For example:

The hand of Fatimah bint Qays was sought by Mu‘ āwiyah, Abu Jahm and Usamah ibn Zayd (may Allah be pleased with them) and the Prophet (may Allah honour and save him) said to her,
“As for Mu‘āwiyah, he is destitute, with no fortune, and as for Abu Jahm, he is a man who beats his women.  Marry Usamah ibn Zayd, though.”
She said, waving her hand dismissively,
“Usamah! Usamah!”
The Prophet said to her,
“Obeying Allah and His messenger is the best thing for you.”
She said, “So, I married him and I was happy.”
(Sahih Muslim 1480)

In this tradition, the Prophet (may Allah honour and save him) advised Fatimah against marrying Abu Jahm and Mu‘āwiyah and mentioned the reasons why they weren’t suitable in that particular context. Was the Prophet “warning” against Mu‘āwiyah and Abu Jahm or simply giving sincere advice to Fatimah? Of course, this does not mean that these two noble companions had issues in their “manhaj” and neither was it “personal”. It was naseehah in a personal affair. Now understand how much more important this is in affairs related to our religion?

Another example:

Abu Dharr (may Allah be pleased with him) relates: I said to Allah’s Messenger,
“Won‘t you commission me?”
He patted my shoulder with his hand and said,
“Abu Dharr, you are a weak man and it is a trust, and on the Day of Resurrection a source of disgrace and regret, except for the one who gives it its due consideration and fulfils his duty to it.”
(Muslim 1825)

And in another narration:
“Abu Dharr, I can see you are weak, and I love for you what I love for myself. Therefore, never oversee even a couple of men and never take charge of an orphan’s assets.”
(Muslim 1826)

This is Abu Dharr (may Allah be pleased with him) an imam in knowledge and asceticism.
Adh-Dhahabi said about him:
“He was a leading example in asceticism, honesty, knowledge and implementation; a staunch proclaimer of the truth, undeterred by anyone’s criticism where Allah’s laws are concerned…”

And yet, the Prophet advised him against seeking leadership because he deemed him unsuitable, and he advised him out of love for his wellbeing, not because he had something personal against him.

In al-Kifayah fi Ilm ar-Riwayah, al-Khateeb al-Baghdadi states:

Chapter on Not Relying on the Authority of One Who Is Not a Person of Precision and Skill Even if He is Known for His Righteousness and Devotion

He then narrates (with chains of authority) from a number of people:

Rabī‘ah ibn Abu Abdur-Rahman who said,
“We have brothers by whose supplications we hope to be blessed, yet whose testimonies we would not accept.”

Yahyá ibn Sa‘īd al-Qatān, who said,
“I could trust a man with a hundred thousand yet not trust his narrations.”

Abu az-Zinād (Abdullah ibn Dhakwān) who said:
“I met a hundred men in Madinah, all of them trustworthy, yet their narrations were not accepted. They would say about such a person, ‘he is not one of its people.’ (i.e. hadith)

Malik ibn Anas (the imam) who said,
“I met seventy men near these pillars”—and he indicated towards the masjid of Allah’s Messenger (may Allah honour and save him)—who would say ‘The Messenger of Allah said’ and I didn’t take a single thing from them—though you could trust any of them over a treasury—because they were not practitioners of this affair.”

And others (see p158 onwards.)

Knowledge has its people, da‘wah has its people, leadership has its people.

And Allah knows best.

Compiled & Translated by Abu Abdullah Nurudeen ibn AbdulAziz.