I personally think the Spanish postal service is pretty good, I live here, and although things go astray it's not often that they do, I've had two items go astray, one turned up eventually, the other was mentioned in an earlier post, not always records as I tend to buy most things on-line.
How could any country claim to know what happens to any mail once it leaves their country? I had more stuff go missing in the UK in the 2 and a bit years I lived there after leaving Canada than I've had anywhere else. Now if mud is to be slung I'd be aiming at Italy, or maybe even France.