Burnham was hampered by not being a Captain, it was like we are told she is the show's lead but it's kind of like you are watching the understudy. Then layer on her messed up characterisation that is wannabe Vulcan alumni and mutinous Starfleet and we end up with a mishmash second rate officer/specialist only part interesting because of her family ties and perverse connections from a mirror universe. She ends up championing more mutiny and threats of mass destruction. But hey, if the Federation does it then that's different, bring on the medal and tedious speech. That is unlikeable.
Least Ed Mercer looked the part, acted the part and made you care.
I do think you might be on to something about Burnham being restricted in a sense since she wasn't the captain. But she's far from a 'second rate' officer. That was established not only in dialogue but by the character's actions throughout the first season. And by the end Burnham was not championing mass destruction. Her intervention stopped a potential genocide of the Klingons, a major change in the character from our introduction in "The Vulcan Hello." The Burnham character wasn't perfect-she wasn't designed to be-but she was interesting to watch than Mercer.
I like Mercer. He is a noble, idealistic, seems like he would be a cool enough captain. I particularly liked the episode where they were trapped on the enemy alien ship and he didn't want to destroy it because there were kids there. That was my favorite show and to me Mercer's finest hour. However, Mercer isn't that interesting from a character standpoint. He's likable but there isn't much change from the beginning of the season to the last. I will give him that he's a little more confident in command, but he hasn't had to lose it all yet, he hasn't hit that wall, and every Trek captain hit the wall, so we'll see if McFarlane puts Mercer through a crucible and how he comes out of it.