Sauron and Istari

[Inziladun]

Originally Posted by Sarumian

Gandalf's battle with the Balrog and Saruman's domination over Isengard and his army are something an "unrestricted" Maya can achieve.

Actually, I think that but for the real incarnations of those two Istari, their efforts in those events could have led to greater success.

The Balrog was not in any way limited in the display of its power, but it's a recurring idea that expenditure of the spirit for evil purposes was a drain on the Valar and Maia. A "chained" Gandalf was still able to beat it, though of course at the cost of his own physical body.

Saruman was inferior to Sauron in power and will, even after the latter's eons-long wasting of his power for evil. Saruman had the limitations, and maybe he would not have fallen under Sauron's sway so easily otherwise, and perhaps could have made his own Ring of Power. Then though, he would have been much more of a threat to the West, as well as to Sauron.
Just speculation, sure, but the "imprisonment" of their spirits in actual mortal bodies was obviously a carefully considered condition when the Valar conceived of the Istari and their mission.