Hunter was saddled with a character starting out as emotionally defeated. We know Hunter had the range and ability to portray a more expressive character, but as written Pike wasn't that way.
Avery Brooks was also saddled with a character who started out as emotionally defeated. "Emissary" is strikingly similar to "The Cage" in the emotional arc for the lead character -- he starts out in pain from a personal loss, tired of his work and considering retirement; he's captured by aliens and interacts with them through illusions taken from his memory; and in the experience, he gains perspective on his problems and comes out of it restored and rededicated to his career.
So the question is, did Brooks suffer from those limitations on his character in the pilot? Or is it maybe not the situation that's to blame? (If there is any blame required.)
John Hoyt was a competent enough actor, but he did strike me as perhaps a bit too old for the role of Boyce (even though I don't know what his actual age was compared to DeForest Kelley).
He was born a bit over 14 years before Kelley. So Hoyt in "The Cage" would've been about 12 years older than Kelley in season 1.
The actor who played Piper in WNMHGB also struck me as not the right type and he didn't impress me at all. DeForest Kelley was indeed the oerfect fit.
Apparently Roddenberry wanted Kelley as the doctor from the start, but the network disagreed. It took three tries to get them to say yes.
In the end NBC not only chose the wrong kind of story for the kind of show they wanted (at least as a pilot to launch the series) but GR and company simply didn't produced a polished enough effort. The second time around allowed them to do it better.
I think "The Cage" is a very good science fiction mini-movie, and my favorite Roddenberry script for the show. But it is perhaps a little rarefied for an action-adventure series pilot.