Bought Assets, whatever they may be, MUST become available to play/install on the user library.
Hiding the assets somewhere almost impossible to find is an extremely questionable practice.
Making the user select one at a time from a huge list is a blatant disrespect for the users time and effort.
The year is 2020, cars drive themselves, rockets land themselves, and a gaming platform can't present some images on a browser/app?
Honestly, what is your excuse other that it going against your profit?
The problem is not a database one, and not a processor scalability one as both are infinitely scalable on any flavours of aws...
So, please be so kind to fix your bug, respect our time, and provide the service you proposed, to deliver ALL the games we bought.
Thank you.

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.

So at this point, how can I easily tell what I have grabbed, and what I have not?  I figured I would leave a tab open, and be able to return to the list and work my way through it.  Unfortunately the list order has changed, and I no longer have any idea where I need to resume from...

Bit the bullet.  Installed the ScriptAutoRunner and used the modded script posted above.  Took a while, even with me being half way through it manually, but it was worth it.

Thanks for the script. Saved a ton of wasted time  Guess I should run Playnite now

For Linux users there is a little Go app here that works: 
https://github.com/tyndyll/itch-claim

I've taken a look at the source and nothing looks sketchy. 

If you trust the pre-compiled binaries, you can download them here: 
https://github.com/tyndyll/itch-claim/releases/tag/v1.0

There are macOS and Windows binaries too.

To build from source, you need git and go installed, then:

$ git clone https://github.com/tyndyll/itch-claim
$ cd itch-claim 
$ go build . 
$ ./itch-claim --username <your-username> --password <your-password> --url <your-unique-bundle-url>

It appears to add one game every 5 seconds. This doesn't seem like an unreasonable load to put on the itch.io servers.