I took 5 or 6 keys from them for different games: 3 or 4 worked right off the bat & 2 did not.
Of the 2 that did not, 1 was replaced some 48 hours afterwards: there are these buttons just below the key on their GA page that ask you if the key worked or not.
However, if the giveaway is suspended, you loose access to them.

If you push the button though which you claim the key did not work, they ask you to make your profile public and search on your profile history page for the attempt of activating the key. If you GA the key, the attempt will not be there, there's no ground to stand on with the claim.

The lesson as I see it is not to get too greedy. If I find a game I like on their site, I will now take only a copy for myself, leave the rest for others. This way, if something goes wrong, I have proof and can go forwards with a complaint.

I don't know how you can see other people's key activations, but you can check yours if you log into Steam on a web browser and then go to
Licenses and product key activations: https://store.steampowered.com/account/licenses/
I think that failed key activations would appear for a limited time only, though I'm unsure what that time limit is.
I had something like "Failed due to..." next to my previous mishap when I checked.