Spencer kept it real. Sam got plenty of love from his fellow heroes. And certainly he had his fans among the public but the political machine wanted to keep him down.
Part of the point that Spencer was making was that much of what Steve was able to take for granted as far as people respecting his stature and leadership, Sam had to fight every inch for. To not be honest about how a black Captain America would be received would be a failure on Spencer's part. His job as a writer isn't to traffic in pure wish fulfillment. Yes, every fan of Sam would to see him lauded as Captain America but the truth is he would meet with a great deal of resistance.
If you don't think that's true, look at the hostile reaction in some segments of fandom when Sam became Cap.
As much as Spencer threw at Sam, though, it only made Sam seem that much more heroic. He had to contend with so much more grief from all sides than Steve ever did and yet still rose to the occasion as best he could.