To be honest, I was quite upset when I found out that the games are hidden. But after sitting on it for a longer time, I don't think itch.io deserves all this hate their getting here. Sure, it's weird. Sure, I think it is a wrong business decision. People think itch.io should potentially rewrite their backend/frontend/app to suite this specific case. Implement paged/delayed loading, eventual consistency or whatever else this would require that they might not have. Sure, all of this is doable and has been dome millions of times. But all of this requires a budget, people allocated to it, potentially expensive new hires to bring in the required experience...
So I do think, that itch is missing out on a good traction magnet - people will be turned off from their client/service. But I don't think customers raging here understand how those decisions are made in software projects. Hell, it might not even be on the table for itch to achieve that due to budget.
And as a developer, I can write my own integration to GOG Galaxy that would just sidestep the itch.io client and integrate the library in some way. Same should apply to Playnite.