Well, again, in Wally's case, he had every right to be upset and uncomfortable around Barry when he was revealed the informations that were kept hidden from him, regarding his father the first time and regarding his "not even meant to exist" thing the second time. While he started to get past his anger over the first matter in time for the Grodd and Negative Flash story's climax, the second bomb shell dropped not long after and opened new wounds. It would be disingenuous for Wally to be made to "get over it" regarding such consistent lack of candid trust and sincerity from Barry, and straight-up disrespectful to drop Wally entirely from the book, even as a recurrer.
Out of universe, these moments coincide with Wally dipping in and out of the story. Williamson is nowhere near as bad as King with his not using Duke, as the former does make use of Wally, and for example the issue where KF learns about his father and that it was kept from him by Barry was a very powerful moment and one of my favorites for the character. King never really even bothered that much. Williamson, though, does flow Wally out of the story, just like he does Godspeed and everyone else other than Barry, only to fold them back in later. I do wish there was a consistent presense, but I can appreciate the story beats better because Williamson does justify (most of the time) Wally's/KF's appearances and absences.
Those who see him as nothing more than whiny in his moments in the Flash book don't pay attention (ignorantly or deliberately) to the times when he's not explicitly angry with Barry, nor do they acknowledge that Wally's times of frustration and abject anger towards Barry and his lack of candidness are justified. Likely part of it is that some of those who do so are sympathetic towards Barry and so won't tolerate anyone against him, regardless of how internally justified the circumstances are. That doesn't make Wally/Kid Flash wrong or whiny, however. And it doesn't justify sweeping such poignant matters under the rug.
What I will say, though, is that I like the idea of Williamson writing back-ups that feature just Wally/Kid Flash doing whatever within his own story. That would be cool, and also keep the character fresh in the Flash book even when he can't be justified by Williamson to be included in the main story.
Lastly, I reiterate that KF's treatment in the Flash book by Williamson is leagues and beyond better than Duke's in King's Batman book(s). Sure, I'd like KF to be in Flash more often and more consecutively, but I can at least appreciate that when KF is there, he's given some form of agency and poignancy. That same appreciation can't be levied towards King's Batman.