Yes there need to be some assumption that your co-workers write reasonable commit message even if they have their lapses now and then.
Another problem with comments in code is that they tend to be short because nobody likes to read code interspersed by walls of prose. But many times if something really needs explaining you also need a little more room doing it. Thus when committing in code people tend to gloss over important detail.
Then of course there is literal programming. The other end of the spectrum, but then we're talking code in documentation not comments in code.