I think Eichel suffers from a couple things, some of which are on him and others that are slightly unfair.
Firstly, he is constantly measured against two other players who have arguably been the 2 D+1 and/or D+2 at the NHL level of any player since Crosby. If you measure him up against someone like Tavares, he is tracking fine. Now, due to McDavid winning the Hart at 19 and being a ppg in an injury-shortened rookie season, and Matthews finishing tied for 2nd in goals while being the first D+1 rookie to lead a previously last place team to the playoffs since the Sun-Belt expansion Eichel is constantly measured against guys who have simply achieved stuff we haven't seen from any other highly hyped teenage center in years. Even going pre-2004 lockout, look at the development curves of guys like Lecavalier and Joe Thornton. Eichel sort of has the misfortune of arriving at the same time as those 2 kids, as they are constant measuring sticks. If it wasn't for these two, Eichel would be talked about much more favorably with more positivity for what he is, than negativity for what he isn't.
The second issue is that Buffalo is a bit of a mess. When he got there, they were a team trying to go from outright tanking, to try to quickly rebuild. The issue with this is, while they added ROR to take some of the responsibility, they had no forward depth throughout the line-up. Due to this, and managing to win now, the team just handed Eichel a boatload of ice-time with limited repercussions for the bad habits you still see in his game. Slow changes, lack of effort on backchecks and making overly complicated high-risk plays instead of the simple ones went unchecked in regards to the leash being pulled in regards to the ice-time. I think the most he ever got was a temporary PP demotion. Now, Eichel is to blame for these habits, but Buffalo did him no favors by not doing more to show that they were unacceptable. At this point, they simply can't use those techniques to the same effect with him having already cashed in. It may have cost them more total points in the standings in 2014/15 and possibly Eichel some points, but it may have helped.
The final issue with Eichel is, I don't think he's an easy guy to gel with playing style wise, not all great players are (its been applied to Crosby multiple times). I believe this is why Eichel can sometimes run into trouble at short international events such as the World Cup of Hockey and the World Championships. It's probably rooted in how he chooses to slow down the game and how puck-dominant he is. In general, he may need similar players to Crosby to succeed, which is hard-working wingers with good IQ's who will win battles and get him the puck. Evander Kane while hard-working, definitely isn't a guy who is high-IQ or a guy who will willingly give up the puck.
If we were picking players to build around tomorrow, and you could choose anyone 22 and under to build around, Eichel is probably still my 3rd choice, even with his flaws and in no way slips past 4 (I'd have it in this order McDavid, and Matthews then Barkov or Eichel). Their is a lot wrong with Buffalo right now, and while Eichel contributes to issues with his occasional defensive gaffs and lack of effort, he is much more part of the solution than a problem. He's had some positives this year, most notably an improved scoring rate at 5v5 and being less PP dependent. Now, I can see why people were disappointed if they expected him to be the Malkin to McDavid's Crosby in regards to ability, but I still think he's among the most talented young centers to be drafted since Crosby, and looks on track to be a John Tavares level player.