DavidGil: I had an odd experience with Impulse myself. Since then, I've never bought anything from there again, though I might buy Elemental when it's released.
Before I move onto the odd experience, I've perhaps bought (either retail versions that use Impulse or digital) around ten games and they all work fine.
So, this brings us onto the odd experience. I own Sins of a Solar Empire and I bought the Entrenchment mini-expansion. Now then, on my main gaming PC, I have it tied to my account. Yet, when I boot up the account on a laptop and it doesn't matter how many times I reinstall the game or Impulse, the expansion is not there at all. However, the base game is still available on the laptop along with every other game. I left one post on the forum to see what was wrong and basically I was asked to contact someone else or do an account switch or something. It was several months ago, so my memory is rather murky. Anyways, I never did so because I'd played the game and well, it's not like it was a full priced game. So, it's not like I can complain seeing as I never did what was asked. It's just odd what happened is all.
So, I'm unsure what actually happened because it seems Entrenchment isn't tied to my account, yet I can still access it on the PC. The only thing I can think of is I changed account details around the time I ordered it I believe, which may have had something to do with it. Actually, there is one other thing. I think Entrenchment could be bought via an external website? I.e. Ironclad and then tied to your Impulse account. I may have done that.
Anyways, I've never experienced the same problem with other places. Occasionally, games go missing on Steam but they do soon return without any emails to tech support.