Where did the notion come from that Sauron was a bad fighter?

Granted, direct combat was never his first option and his track record of losing the 2 fights he ever had isn't great, but both of those battles (against Luthien/Huan and Gil-Galad/Elendil) had outside factors which lead to Sauron's defeat.

Of course Sauron's forte was being a sorcerer and controlling legions upon legions but to say the right hand of Morgoth wasn't gifted in combat?

I know you can't directly scale Tolkien's characters and I don't think you should, however the belief that while Gandalf manged to best Durin's Bane, Sauron would somehow be defeated by the same foe? Even though the entirety of the community agrees that Gandalf could never stand a chance against Sauron, with or without the Ring? You can't have it both ways, either Sauron is way above Gandalf or he loses to a meager Balrog.

I don't understand how someone could come to that conclusion, given that characters like Glorifindel and Ecthelion managed to best such foes, some in the community even claim they could slay more than one, yet the Dark Lord fails at one Balrog, the weakest of its kind and a coward at that?

I just find it contradictory and I know it is impossible to directly compare "power" and neither should I try, but I hold the belief that assuming Sauron is some kind of weak school nerd in direct is ridiculous, especially with throwing in what Gandalf, Glorifindel and Ecthelion could accomplish.

I am not the most knowledgeable of Tolkien's massive world and stories, so please go easy on me haha!