So it seems to me that you are actually talking about a specific fighting group, Roger's Rangers. They were commanded by Robert Rogers, who founded them during the French and Indian War, and were an effective fighting force in the rough terrain of colonial America. Roger penned “The Rules of Ranging” which are still in use by US Rangers, in a modernize form.

After the war, Rogers had a rather fascinating life that was shared between both continents and involved him writing two books as well as an expedition in search of the Northwest Passage. Financial problems marred him and he was thrown into debtors’ prison. He had drinking problems, ambitions of power, and was even charged with treason against the crown for scheming with the French. After mostly clearing his name, he spent some time in England where he ended up in debtors’ prison again. He sued for false imprisonment and other possible damages but dropped when offered a major’s half-pay.

He returned to America in 1775 with the Revolution in full swing. He was first arrested as a possible spy and released with the promise that he would not use his services against the colonies. He was offered a commission but turned that down to then offer his services to Washington himself. Maybe it was his character or his history but we can't be certain why he was rejected. What is certain is that he Washington had him arrested only for him to escape and join up with Loyalist forces. He formed both the Queen’s Rangers and the King’s Rangers who fought against Patriot forces.

Not all of the Rangers followed his lead. Two examples were Israel Putnam and John Stark, both who were present during the Battle of Bunker Hill. Both of them gained fame and distinction and were trusted.

So it's not so much that the Ranger's weren't trusted, just that not all of them were trustworthy.

I hope this answers your questions. Here’s a quick biography on Roger’s life if you are interested.