rz4: Please GoG continue to support your downloader.
Two things I want to say to you:
1. I don't share your experience. I don't use Galaxy, and I haven't had failed GOG installer downloads for ages. Nowadays I mostly use the gogrepo.py tool, but to my understanding it uses the same commands as downloading with a web browser.
2. You said yourself that your Riddick download failed also with the legacy GOG downloader... so why do you want GOG to keep supporting it? When I had problems downloading GOG games (they would get corrupted quite often, that was when I tried to download them in Thailand over a shitty mobile internet connection)... it was that same Downloader which brought those corrupted downloads to me.
So whatever on-the-fly integrity checking there is with the legacy Downloader, it seems far from perfect.
Come to think of it, I am not even convinced using Galaxy would necessarily bring you uncorrupted downloads either, if you are having such problems often. Just a hunch. Either there is something seriously wrong with your internet connection (like was for me with a slow mobile GPRS connection in Thailand), or could even your hard drive be breaking up, corrupting stuff you download?
That said, I wouldn't say no to some kind of p2p client that allowed downloading games from GOG, as to my experience they have the best success rate providing uncorrupted downloads for some reason, even over shitty slow connections. They just seem to catch faulty downloads actively, and re-download the corrupted parts automatically.