Cleveland was a very good team, despite a lack of elite talent other than Lebron, that wasn't able to get over the hump. I don't think they ever had issues like Miami is seeing now, and I also don't think they really underachieved much.
This Miami team is just in a tough position. They have some pieces that are not perfect fits, and a lot of other pieces that are simply missing (no quality PG, C, or depth in general).
While things are not going well there right now, I just think its way too early to be blaming Lebron's ego. They definitely have some PR problems, but on the court, it has much more to do with a lack of leadership (coach needs to have more control), and lack of a complete team, than anything else.