There's no real harm in a Not Received; stuff happens and people recognise that.

You can avoid them (as I have with a couple of thousand giveaways) with a little diligence (e.g. using trustworthy key suppliers with support staff who will address problems with keys, and of course keeping proper track of your keys).

Before I started making giveaways, I had hundreds of spare keys which I hadn't kept proper track of, and gave them away in gaming chat rooms, etc. Then I joined this site and gave them away on the Orphaned Keys discussion thread. Once I got to the point where I was confident in the provenance of my remaining keys (and only then), I started doing actual giveaways. As I'm not very good at keeping track of things myself, I simplify things by immediately redeeming all Steam keys that I want, and creating giveaways for those I don't (absolutely no need for any text file or spreadsheet to track unused keys!). I create almost exclusively very short giveaways, firstly because I'm a little impatient myself, and I know other people might be too, but also because if there's a key issue I will generally find out quickly enough so that I either ask the supplier's support to rectify the issue, or obtain another key while it's still cheaply available (though never actually had to do that... yet!). I have no doubt that one day I won't be able to resolve a key issue with one of my giveaways, and the cost of fixing it myself (having to buy a full price key) won't be worthwhile, or the keys may just not be available any more. But so far so good...

Finally, whilst I don't expect any leniency from people I may disappoint even though I do try to be diligent, I do make exceptions myself for gifters who clearly have a history of being diligent and have a problem which they can't realistically fix (these people will typically have hundreds or thousands of giveaways behind them and an unblemished record, and will have shown at least an effort to resolve issues). However, in general I think it's best for the SteamGifts system to accurately reflect a member's record.

I wish you all the best with your future giveaways, and hope that this reflection on my experience here was somewhat useful.