A few weeks ago I had a windows installer that was silently failing when upgrading from an older version (installation from scratch was working without issues). And as windows install logs aren't exactly easy to read, I was stumped, until I took an upgrade log from an older, working build, strip all date information from both files and compare them, checking all the sections which were different, until I found a line indicating that a colleague had forgotten about a limitation when dealing with msp (don't delete components on minor upgrades, but I didn't throw any stones as I've done the same mistake, twice, one and two years ago...)