I have a new Head Canon: Incorporating a Maia into a human-ish body makes them crazy.
Two Blue Wizards: Instead of doing their job they disappear into the East. (My favorite theory is they end up in Japan and create bonsai.)
Radagast: The most recent edition of Beyond Bree contains a nice summary by Wil Korver on Radagast's failure to do anything substantial to support Yavanna's chosen (Ents & Entwives) as well as his inaction on Sauron. ("For the Birds: The Arrival of Radagast, Saruman and Gandalf to Middle Earth" Beyond Bree June 2020).
Saruman: If Curumo had really wanted to learn about ring-making, he had from SA1697 to TA1000 or so (2,700+ years) to stroll over to the Halls of Mandos and ask Celebrimbor. If he had taken the Radagast-insanity route, he would have spent his time puttering around in his home forge in Orthanc, not building an army mirroring Sauron. If Saruman had simply despaired of his ability to add value to the fight against Sauron, the sensible thing would have been to write a suicide note, jump off the top of the tower (spiked wheel landing optional), skedaddle back to the Valar and let them know that a second darkness was nigh. If Saruman had wanted to be THE Dark Lord TM, the sensible thing to do would be to wait for the West to defeat Sauron, take excellent notes, wait till the parties were over and everyone went home, then start the ever-so-pleasant task of building up his Evil Empire exactly how he wanted it, applying everything he learned from Sauron's demise (insert visions of industrial revolution slums and WWI battlefields here).
Why not Gandalf? My theory is that, while Curumo was chatting up Celebrimbor over ring lore (if he did), Olorin was having tea and crumpets with Melian about how to maximize his ability to be effective as a Maia in human-ish form. Melian, delighted that someone finally wanted to actually listen to her, gave him the straight poop.
My proposal going forward (yes, I have one) is to pay close attention to what Saruman does, and very little to what he says, in order to figure out what his end-game was, because, as I understand it, it makes zero sense. He seriously wants to be éminence grise to/for Sauron (until he can stick a dagger in his back some dark and stormy night)? AND he thinks that the PowersThatBe TM are going to allow Eru's 2nd batch of children to be killed/enslaved by Sauron in Melkor Part II, when they didn't put up with it the first time? Not buying it friends.