Sorry for the addition OP but I think there's a weird dynamic with some fans where they take the TV show canon and then read TLG and so they see Ilya's more positive seeming relationship with the Raiders (compared to the Bears, which are described as being hard partiers and with whom Ilya seems to have no intimate friendships) and his close friend Svetlana in Boston and they see it as Ilya having lost so much, and Shane ignoring it.
But book!Ilya says the Bears kind of sucked. He genuinely likes the Centaurs a lot more. Book!Ilya doesn't have childhood friend Svetlana, she's just an old hookup he likes better than most because she's Russian and knows puck. Book!Ilya wasn't leaving behind anything in Boston other than his career.
Which is a lot! Obviously! But it's a calculation book!Ilya makes very deliberately.
The book spells this all out pretty clearly too. The conflict Ilya has in TLG is he doesn't know how to be in relationships---platonic, romantic, AND familial. He doesn't know how to express his needs to Shane. He doesn't know how to build intimacy with friends. And he doesn't know to be part of a family dynamic. He learns each of these over the course of the book, with the help of his therapist.
Shane's shittiest moment in TLG is when he accuses Ilya of just slacking off and smoking weed, but if we think about it from Shane's perspective for a hot minute, this is when Shane's out to the Voyageurs and things are pretty obviously not going well, based on how fast stuff falls apart after the leak. Shane's developing an eating disorder and Ilya won't stop bitching at him about his diet, he's worried about Ilya but Ilya won't talk to him, he's (in his mind) running out of time to acheive his career goals with his 30th bearing down on him.
Whereas (from Shane's perspective) Ilya is having a ball on the Centaurs. He's throwing fun Halloween parties. His house smells like marijuana. He's going to gay bars with his hot gay available teammate Troy Barrett. He doesn't seem to care that the team loses constantly even though wasn't it hockey that they fell in love over? Doesn't he miss competing with Shane? Why won't he talk to him?
So Shane's over here like "I thought we were doing this together for our careers and you're fucking around and having fun. And now you're asking me to put our whole plan in jeopardy for what, a party? What the fuck?"
Anyway my point being that I don't think either of them are in the wrong they just don't know how to communicate because they don't know how to be in a relationship because they're barely adults and have never done any of this before and they're doing it under the most stressful possible conditions. I love how Yuna says she's proud of them because really it is a miracle what they manage to do and how they manage to love each other despite everything. Or not a miracle, they fucking work for it.