I think part of the reason they are compared is that both get a significant portion of their value from Defense. Also, Andruw was frustrating as a hitter, not because he wasn't productive, but because everyone thought he could have been even better. Likewise, watching Heyward hit, you think he must be ready to break out at any moment. He looks like he ought to be really good. Expectations are very high and performance has, so far not lived up to them.
Which is a human perception problem which I will admit is sometimes hard to get out of the way.
Player A wasn't much in the minors but overcomes it and becomes a good player.
Player B was all-world prospect but merely becomes a good player in the majors.
It's not hard to look at that and consider Player B a disappointment while Player A is a success. Even if they are both quality big league players. At some point you have to judge players on what they are actually doing on the field and not what their expectations were as a teenager or young adult.