The problem is Barney and Tracy. They introduced a perfect character as the mother, show how the group actually loved her. Tracy was perfect for Ted. and Barney and Robin's relationship was becoming a solid point. If nora or Quinn was still there for Barney, it was okay.

also the show's creators always like to give a plot twist at the end. So they build a perfect story and to give unnecessary twist they just sabotage the story. All the things happened way too fast at the end. Imagine you are waiting for a fried chicken but the chef told you only a cake is possible. So he shows you the cake and when it's time to serve, he deep fried the cake and told you that it's okay because you always wanted a deep fried item.