The difference with Vik is clear, he went to a top European side, playing European football, with potential to win trophies (and with him, they did - won the league) and play in the Champions League. It was a clear and obvious step up. Also, he's gone to the right place to prove himself and ensure his next move will be to a massive team - Arsenal should have signed him last summer.
Gus went to the second worst premier team of all time, playing turgid football. He still managed to get something like 5 goals and 7 assists at that level in a dog shit team, but it isn't the clear and obvious step up he would have wanted, and that we all thought he deserved - if he was going to go. It's a shame he didn't go to a better side, I think he would have fared much better in a better side playing good football, and none of us, except the most bitter, could complain if he had gone to a Fulham, Brighton, Brentford etc. I certainly think he is good enough to do so, and would have a better platform to showcase his considerable talent.
We still got good money for both, arguable the most we could have given their contract situations. But the perceived lack of a clear step up for Gus and Cal are what makes it more aggravating, throw in the nature of O'Hare's departure, and you can understand the grievances some fans have with him. Albeit harsh when Gus is still mostly lauded, and did kind of the same but different in terms of how they left. Although Hamer's contribution to the club is borderline legend status, he singlehandedly dragged us to and through those playoffs at the end of that season when Vik was playing through injury, and had he not gone off injured, I think anyone who was in the ground felt we were going to win that in normal time, let alone ET.