Humble doesn't change stats, if just creates a "plain" character at the time of use. So, the Scavenger retained its 28/28, but becomes a vanilla creature with no abilities.
If you used Humble on the Scavenger when it was a 2/2, it wouldn't have been able to grow due to losing its ability.
If you want the best rule of thumb when figuring out how humble will affect a character, just think about the character becoming a vanilla character (no special abilities) that retains whatever stats it has at the time you use humble.
(So yes, it does away with flying, unstoppable, multi strike, etc...any special effect)
You are right, though...some of these cards do need to be more clear. It is not unreasonable to expect that the Scavenger would return to its original value based on the way Humble reads.