Unfinished Tales question

J.R.R.Tolkien's subcreation was always a living constantly developing structure.
And he obviously changed his mind about many things over time. In The History of Middle-earth publications this process is very well represented.
I think the best way to look at Radagast is through Gandalf's words:

"Radagast is, of course, a worthy Wizard, a master of shapes and changes of hue; and he has much lore of herbs and beasts, and birds are especially his friends."

"At first I feared, as Saruman no doubt intended, that Radagast had also fallen. Yet I had caught no hint of anything wrong in his voice or in his eye at our meeting. If I had, I should never have gone to Isengard, or I should have gone more warily. So Saruman guessed, and he had concealed his mind and deceived his messenger. It would have been useless in any case to try and win over the honest Radagast to treachery. He sought me in good faith, and so persuaded me.
`That was the undoing of Saruman's plot. For Radagast knew no reason why he should not do as I asked; and he rode away towards Mirkwood where he had many friends of old."

So Radagast is worthy and honest. And Gandalf was rescued from Orthanc thanks to him. May be he avoided the events of the war, but he never turned to evil.

One could also consider that given that Saruman was prone to abuse Nature and exploit creatures, if they had not had the alternative experience of a supportive 'wizard' in Radagast, the wild things might have become more embittered and hostile, and aggressive against the Nine Walkers.

I think Saruman and Radagst were the only ones who failed. The other two went east to end Sauron's threat while Gandalf remained in Middle Earth.

While it's true that he did help to rescue Gandalf from Orthanc, I think he did fail in the quest the Valar gave him, actively to help against Sauron. It's significant that we only hear of him once or twice during The Lord of the Rings, and only near the beginning; he doesn't help in the war, when presumably he could have, and doesn't even send beasts and birds to help either.

I remember hearing once (from Corey Olson) that Tolkien actually changed his mind on this over the years. The reference in Unfinished Tales was an earlier essay where Tolkien really played up the role of Gandalf while downplaying the other wizards. Later on in his writings (found in the Peoples of Middle Earth?) Tolkien softened a bit on this. In this later version the maia who would become Radagast was sent over by Yvanna with the maia who would later become Saruman. It would seem here that Ragadast's quest was one more based in protecting the natural world from corruption (including Saruman's ambition). Pardon the lack of direct references.

Radagast also kept an eye on Dol Guldor during the Watchful Peace. I think it's fairer to say that he did his part using the skills he had, but he didn't accomplish as much as Gandalf.

I recollect that Christopher Tolkien suggested that Radagast's "failure" may not be as it seems as he was assigned to protect the flora and fauna of Middle Earth and that mission extended well after the defeat of Sauron. I forge the name of the one who chose him but the suggestion Christopher made was that he/she/it might have given him that exact assignment and thus why he is never heard from again.